Yeah, you should be able to do that, but it will overlap the memory space reserved for the MegaCD or 32x. All that means is you cannot use them while using that cart.
I will draw a diagram for you later.

All you have to do is tap into the higher address bits on the cart edge which go to the 74ls139, then the outputs from the 139 go to the the control logic inputs (Chip Enable) of all of your ROMs.

Read the datasheet!
http://pdf1.alldatasheet.com/datashe...I/74LS139.html