This won't work.
The vfat boot partition should contain only boot files, and the linux OS has to run on a linux FS type. Period.
What you could do is shrinking the ext partition, then create a new one, format it VFAT and mount it on /home/pi/roms, but this would cause issues with simlink ans so, and it's really FAR from being straightforward.
SMB and USB should not be so slow, especially if you only need 2Gb, and I would recommand to use them.