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.
@qu9ke RetropPie will not use the FAT32 formatted partition to store the data. It doesn't matter if your SD card is formatted or not. If you write the RetroPie image on the card, you'll see that will create 2 partitions
a small Fat32 partition to hold the boot files and boot configuration
a larger Linux partition that will actually hold the system, including the ROMs, emulators, etc. This partition will be automatically expanded on 1st boot to fill in the rest of your SD card.
So yes, you're overthinking it. Just write the RetroPie image on the SD card and start your Raspberry Pi.
So I played around a bit, got a new SD card with the image on it.. did all the typical tweaks (enable wifi, disable overscan). I then followed the guide up to editing the fstab. Plugged in the UUID appropriate for my drive, and used this command:
Essentially, I'm using the vfat example in the guide, and just changed it to ntfs. The rest of the options I'm not sure what are for, but it looks like user permissions (like read/write/execute/etc) for the path and it's specifying the user as pi so I left that in there from the example. I'm not sure about the umask bit, so I just left it too. If somebody can clarify on what all those are, I'll edit my setting if needed. But for the most part, it looks like it's working. I was SFTP'd into the Pi prior to rebooting. I could access /home/pi and everything else on the system just fine, but not /home/pi/retropie. However, after rebooting, that last /retropie/ bit opened up and gave me access to things like the bios and roms subfolders. So I'm assuming that means the automount DID work. The only way I'll know for sure is if I transfer in bunch of files over 8gb, because that's the limit of my SD card. :)