I think, I figured out the root cause of this.  GApplication launches
apps through dbus, looking up the respective service in share/dbus-1. 
The service for GNOME Maps does not start the actual program, but much
rather a gjs wrapper, which we don't patch -- and which we can't simply
patch, because that leads to other errors, that rival our own when it
comes to how cryptic they are.

I've attached a patch, in which I rewrite the DBUS service to use the
actual program.  I have not fully tested this patch, as `guix
environment --ad-hoc gnome-maps` appears not to be strong enough to
influence the gapplication lookup.  However, if you launch `gnome-maps
--gapplication-service` in one terminal, then quickly switch to some
other and launch `gnome-maps` normally, the one that spawned the
service is the one that gets used, so claiming the bus in this fashion
*should* work.

Regards,
Leo