Just as a test, I enabled user specified environments in sshd_config (such that ssh reads .ssh/environment) and added the following there:
PATH=/gnu/store/0awhym5h0m890n0wq87y0dxznh14rk88-guile-next-3.0.1/bin:/home/guix/.guix-profile/bin:/usr/local/bin:/usr/bin:/bin:/usr/games
GUILE_LOAD_COMPILED_PATH=/gnu/store/nkh7c4ygaivfxdq3zhccl4a5qxrn6s88-guix-module-union/lib/guile/3.0/site-ccache:/gnu/store/0awhym5h0m890n0wq87y0dxznh14rk88-guile-next-3.0.1/lib/guile/3.0/ccache
GUILE_LOAD_PATH=/gnu/store/nkh7c4ygaivfxdq3zhccl4a5qxrn6s88-guix-module-union/share/guile/site/3.0:/gnu/store/0awhym5h0m890n0wq87y0dxznh14rk88-guile-next-3.0.1/share/guile/3.0
GUIX_PROFILE=/home/guix/.guix-profile
GUIX_LOCPATH=/home/guix/.guix-profile/lib/locale
This made the offload test work.
Crucial here is the PATH to guile 3.0.1 as well as the GUILE_LOAD_* paths.
But how are these *supposed* to be setup on the build host???