Check mobile home supply stores. Some of their toilets have bowls that are smaller than typical.

"Check mobile home supply stores. Some of their toilets have bowls that are smaller than typical."

And may not be listed for anything but a mobile home.

The standards are NOT the same.

Some in wall toilet flushing system vendors are
Geberit
Grohe
Toto

Geberit has a larger opening for the flushing actuator than Grohe.

The typical system assumes a wall depth of ~ 6". There are systems that can be used with a wall depth of ~ 4".

The bowl is separate.

Installation is more involved than the standard toilet. The height has to be set when the frame is installed. You may have to move the vent stack, put in extra framing besides re-routing the water supply.

The mechanism works well - very quiet and efficient.

Just don't puncture the tank.

I did find a few toilets that were more in the 26 inch range and a few that were actually comfort height as well. I liked the in-wall systems I saw online, particularly the Geberit. If my plumber buddy can help me with this I am going to try it, otherwise I will go with the 26 1/4 deep one I found that is ADA height with 3 inch flush valve. This will save me 3 inches which will help with shower door swing which I wood like to be 180 degrees (open in and out).

Other things to think about
1. The replacement toilet flapper and other parts (other than the supply line) must come from the manufacturer of the in wall unit. It will be quite a while before things fail though.

2. The bowl mounts best on a tiled wall, not drywall.

3. A lot of pre-planning will be needed especially since you will need to modify the DWV, framing and floor. Plan on getting the calculations for the toilet height and discharge pipes (bowl connection) correct the first time round.

Having said that, the results are well worth it.

You can always use flushometer valves and a wall hung toilet.

We installed a duravit toilet with a geberit mounting system. We put it into a 6" deep wall. Its great; quiet, takes up less space and since there is nothing below the bowl it helps keep a very small powder room feeling more open and less cluttered. The 6" deep wall allowed us to tile in a wall niche above the tank where we put in glass shelves that gives us a little extra storage in a tiny room. Not cheap but worth the extra space it offers.