Hi Eric,
If there is access from the basement below most of the time door alarm sensors are easy to wire even on an older home. French doors usually have one door that must be open before the other will move. Some older models allow either door to operate independently of the other. If yours are of the former type you only need to put sensors (called "contacts") on the door that moves first.
The simplest way to protect French doors is with a roller/plunger type sensor installed in the frame on the hinge side. I've tried numerous brands and found that most tend to jam shut after a few years. The one exception, IME, is a model from Sentrol (now part of GE Security) called a 3005. When installed properly about the only thing that can present a problem for these is paint. Just don't paint them. This is how I've done it a few thousand times.
Using a 3/4" speed bore, carefully drill a hole in the hinge side of the door frame, 2" above the threshold and parallel to the floor. Make the hole about 2" deep. STOP.
Swap the drill bit for a 1/2" diameter, 16" long bit (often called a "feeler bit"). Place the tip of the small bit inside the large hole. Raise the drill motor so that the bit is about
60º off the floor. Swing the motor outside the door about 3" so that the small hole will appear in the basement. By drilling at this compound angle you won't wind up going through a 10-12" joist.
Mark the cable with a fine point, Sharpie laundry marker and shove a few feet of it into the hole. I like to use 22-gauge,
4-conductor (22/4) solid cable for alarm sensors. The jacket is just a little smaller than that 1/4" hole you just drilled and the solid core stuff is easy to push through to the cellar below. Have your SO pull enough cable out in the basement to make neat turns over to where you will hang the control panel cabinet, leaving a couple of feet extra at each end. Bend the extra pair of wires out of the way but don't cut them off. They'll come in handy if you accidentally cut or short another wire with a staple.
Solder and tape or heat shrink the connections at the sensor end, place a tiny dab of silicone sealant (RTV or bathtub caulk is OK) on the flange of the sensor and shove it into the hole. There are screw holes to mount the sensor but I prefer a dab of RTV.
Leave a 12-18" "service loop" of cable in the cellar below in case you ever need to change the sensor.
Most US homes built in the 1920's are of "box sill" construction. That means the wall is open from the bottom of each first floor window to the top of the foundation. Joists rest directly on the foundation or, in some cases, on top of a double layer of boards called a sill plate which is laid flat atop the foundation. At the ends of the joists a board is nailed in place to close the outside. Short, horizontal boards called fire stops are placed between the joists in the basement.
Very few houses from the 1920's have adequate insulation. Some have had it blown in later. Some have nothing at all. No matter which you have it's not hard to fish cables through the outside walls.
Keypads, motion detectors and such usually use the same
4-conductor wire mentioned above. Make a small hole in the wall where the device will be installed and shove a little cable inside. If you meet no resistance at all it's probably uninsulated. Shove enough cable in to reach the floor plus an extra couple of feet.
In the basement, use a 3/4" speed bore (sometimes called a "paddle bit" to drill a hole in the fire stop below the detector or keypad location. Make the hole at about a 45º angle so it's easy to feed an electrician's snake into the wall. I like to mark the snake a foot or two longer than the height of the cable.
Push the snake up until the mark reaches the hole and slowly pull it back, making a couple of turns with the handle as you go to snag the cable. Once you have the cable the rest is easy.
Note: Some older US homes have diagonal hurricane braces installed between the last two studs next to each corner. In particular if the house is in New England and it was built in the late 20's to early 30's avoid corner locations.
Also, some homes have horizontal fire stops midway up the walls. This was not too common in the 1920's though it is fairly common on more recent construction.
With a little patience and a modicum of tool skills, most people can install a hard-wired security system in a weekend or two. The advantages of hard-wired are threefold. It's not likely to become obsolete in your lifetime. It requires less maintenance. There aren't a lot of boxes on the walls (makes for a higher WAF rating).
Installing four or five door sensors in a 1920 era home should only take a few hours. The keypad and siren(s) shouldn't be a major problem if you avoid putting them right next to a corner. There are flush-mount, interior sirens that can be installed at heater vent height, a few inches above the baseboard. They look like air return grills only smaller.
If you want to place a keypad on the second floor, go to the attic and look for the DWV stack. That's a black pipe sticking up through the roof to vent the "poop pipe". In the 1920's no one had large hole saws since they didn't yet exist. The DWV stack was fed through a square hole cut in the top and bottom of each level, leaving a handy void (no pun intended) where you can run cables for an attic or outside siren, second floor keypad, smoke detectors, etc.
Feeding down to a bedroom keypad from the attic is easy. Look for a flat 2x4 board. That's the top of the wall. Wires emerging from that board near the bedroom door are from the light switch. Drill a 3/4" hole an inch or two to one side from the
110 VAC stuff with your paddle bit and drop the wire in the hole.
If you need more help planning or picking out a system, contact me if you like. I sell this stuff to DIYers in an online store in case you're interested.