I'm also stuck with this issue trying to get LB running GB games with RA set up as emulator and i figured out that if i set the command line core association f.e. -L "cores\gambatte_libretro.dll" as default command-line parameter it works.
so i was trying to paste this parameter to the associated platforms tab as an extra command-line parameter per core but it didn't work - it won't save the given input in this field and hence it won't start the game. It only worked as described above, which is just a workaround for me since it would start only one core.
Maybe somebody has an idea how to solve this and maybe steveaside will get the same behavior...