It's possible to make a plugin that is cross platform in this case as well.
What you do is the following (this will only work in the next release of calibre)
Put all three versions of the dependency in the plugin zip file, alongwith any needed dlls. Put them in subdirectories called
windows, osx, linux
Then in the run method of your plugin do this:
Code:
from calibre.constants import iswindows, isosx
pdir = 'windows' if iswindows else 'osx' if isosx else 'linux'
ppath = os.path.join(self.sys_insertion_path, pdir)
sys.insert(0, ppath)
try:
import your_dependency
plugin code
finally:
sys.path.remove(ppath)