>I hadn't realized that other languages in Guile might compile to Tree-IL 
directly instead of generating syntax objects. Is that common and/or 
encouraged?

I haven’t really checked, but probably? For anything that’s not Schemy in its macros, it strips a pointless layer of abstraction, and Tree-IL is straightforward to work with.

>It seems like it would require the new language's compiler 
to do a lot of work that could otherwise be delegated to Guile, and it 
would make it difficult to implement parts of the new language using macros.

I don’t see what this lot of extra work this would be (unless the language has macros that are Schemy). But yes, if implementing terms in macros, it seems pretty inevitable to use syntax objects, in which case I suppose you might compile to Scheme instead of Tree-IL.

For the mentioned IDE, it wouldn’t really matter what’s happening precisely, as long are there is a path from ‘language -> Tree-Il’.

Best regards,
Maxime Devos.