I'll look into using the value defined in the board.txt for var megalite.build.core as the subfolder in /hardware/arduino/cores/.
But don't expect it soon as my robot needs so much time.
Don't worry too much about it, now i have a complete arduino instalation for each core and just change path in preferences-Arduino, so i have it working and looks that this is not very useful for most users.
Have tested the plugin with several small projects and it works very well.
Best regards.