Most terminal emulators have no way of reporting their capabilities at run time. And having dtach or abduco modify the TERM environment variable of the already

Most terminal emulators have no way of reporting their capabilities at run time. And having dtach or abduco modify the TERM environment variable of the already spawned child is not possible, and even if it did, there is no standard way to get the child to reread it. So, no, the tech is kinda broken. The only correct thing to do is to have the attaching program also do terminal emulation (or, assume that you always attach with a compatible terminal). You could make it a simpler form of emulation, and then have the more complex muxing ability as a separate program, at the cost of having to go through multiple emulation layers.