>> It seems you have to set LDTL_LIBRARY_PATH.
Typo! I mean LTDL_LIBRARY_PATH/
> I saw that, of course, but I’m not sure what that variable even is.
> In particular, I have no reason to believe that it affects Guile’s
> load path...but I’ll try it and see.
It's documented in the guile manual, being the path for extensions:
A shared library can be loaded into a running Guile process with
the function `load-extension'. [...]
For this to work, `load-extension' [...] will look in the places
that are usual for your operating system, and it will additionally
look into the directories listed in the `LTDL_LIBRARY_PATH'
environment variable.
Werner