I'm not sure why it would have any such limitation. Maybe it's too slow for the Maria structures, but I doubt it. It's a modern surface mounted 128KB SRAM chip so I assume it's at least as fast as the 80's RAM in the console.
That all might be true but if I remember correctly(as it was a while ago),I asked the developer of the unit if it could use
the extra as Maria ram and I believe he told me it was to emulte ROM only. Perhaps he did not hookup the write line
to the 7800.
The Maria always just reads anyway, it never writes (like Mord was saying).
But I suddenly remembered something and just checked the programming specs. I'm guessing this is what the developer would have been talking about:
...<holey-DMA blah blah>...
MARIA can be told to interpret odd 4K blocks as zeros, for 16 high zones, or odd 2K blocks as zeros for 8 high zones. This will only work for addresses above x '8000'.
So holey DMA won't work in addresses below $8000 - and the XBoard RAM (and all carts that have RAM) sits below that. So if you need holey DMA then you can't put those graphics below $8000.