Idea uses the system variable idea.plugins.path to specify the location of
the plugins directory, which can be specified in the idea.lax file (or on
the command line if you don't use the lax file).
I have noticed that the SpellCheck plugin doesn't work when you do this. It
looks for the plugins directory to be in the plugins subdirectory of the
IDEA installation directory.
This would be an extremely easy mistake since IDEA doesn't really advertise
the system variables that can be defined in the lax file (or on the command
line). They can, however, be found here: news:email@example.com