LADSOA_PATH may include up to several system and one user paths for plugins. But currently it checks only first path, which in my case is user's.
It worked for me to pass multiple (existing) LADSPA paths with the LADSPA_PATH environment variable (separated by colon on Linux/Mac, semi-colon on Windows) with a recent version of LinuxSampler. My guess is that you probably had one invalid path in your LADSPA_PATH variable.
I just fixed the latter problem (SVN r2509). So that LinuxSampler does not stop searching for LADSPA plugins when one LADSPA path was invalid/non-existent. So it should work for you now with latest SVN version.
Closing this report now. Thanks!