So I tried to add the *helm*/lib/lv2 path directly in reaper, without using any sessionVariables, rescanned, and the plugins is detected:
I used the full absolute path here because I was testing it in a nix shell, but you should be able to something like /home/yourname/.nix-profile/lib/lv2 (double check if the folder helm is present in case home manager puts it somewhere else… also, avoid using ~/ as a shortcut for your home as I’m not sure if reaper can interpret it).
Then, it seems like I can add this virtual instrument as expected:
Is it what you are trying to do? If not can you explain more precisely what you are aiming after?
If you want to use (LX)VST instead of LV2, the path is lib/lxvst instead of lib/lv2:
$ ls /nix/store/b98w26ixa4adx8v44czd08clk1bjxb82-helm-0.9.0/lib
lv2 lxvst
Demo (again, don’t use the path to the store, but something like /home/yourname/.nix-profile/lib/lxvst):
and it appears as expected:
I can then insert a new midi item, edit in in the builtin midi editor, and enjoy my keyboard:
As far as I understand, the bin folder is of no help in that case as you don’t want to run helm standalone, but as a plugin in reaper.