Would it be difficult to implement something like /axmerge <filenmame>. To merge another layout into what is currently present on the server. Basically like a /axload but with no implicit clearing.

Look at documentation of LYT.
https://www.lfs.net/programmer/lyt

Should be quite simple to merge two LYT files into one.

Sure that's one way but very inflexible and creates a lot of similar copies nobody really can keep track.
It would a lot easier to do something like
/axload basic_layout
/axmerge feature1
/axmerge feature23

Quote from chucknorris :

It would a lot easier to do something like
/axload basic_layout
/axmerge feature1
/axmerge feature23

It is possible to replicate that exact functionality using InSim.

I wrote an offline tool to do this when open configs first came out. I've been meaning to merge it (no pun intended) into an InSim app, but haven't got around to it yet.

https://www.youtube.com/watch?v=hp09K_YcYRA

But those DIY solution are at most workarounds. Especially when it comes to format changes like it happened just now with version H.
LFS itself does know how to treat old layout formats.

Also, managing layouts via insim brings up yet another problem. You cannot ask for the objects that are currently loaded. Insim only knows of changes that were made AFTER the Insim app has connected. I once asked for that functionality years ago but it was turned down.

So bottom line, a function that is accessible for non-geeks and non-programmers can't be all that wrong, especially as it is a 99.99% copy of an already existing function.