having relays on the 'main board' will always be problematic if the contacts carry anything significant..

The relay coils (when equipped with flyback diodes) rarely are a problem unless the realys are huge, but if the contacts carry anything significant that is where you will get the problem.

So, if the contacts (as should be) are not connected anywhere on the board I (in the unlikey event that I would put relays carrying anything more than a mA or two) on the main board) would let the ground plane be below everything.

Erik