Long story short-- I had a bunch of Seibu SPI motherboards from various regions and a bunch of carts from other various regions and I decided to switch everything to "USA" so we can run them in the arcade and people can actually read the text.
Anyway-- if you need to convert an SPI motherboard to a different region, you need to remove U1053 on the motherboard (over on the right edge-- it usually has a white dot on it). It will typically be a Sharp LH28F008SAT or an Intel E28F008SA. The first byte of the Flash is the region code. 0x10 is USA. (Others I've seen are 0x80, 0x24 and a couple others-- I can post what went where if anyone needs the info.)
What I did was use MAME to generate an "rfjetu.nv" image. I trimmed off the first 0x200 bytes (looked like "other" novram crap from MAME) and then split the remaining 2048K in to two, 1024K files. The first half of the data is U1053, the second is U1054. I then just used the U1053 image to program up new E28F008SC chips (the multivolt version of the older E28F008SA's) and install them back on the motherboards at U1053. I'm 99% sure that you could just change the first byte in the image though too.
The original boards tend to have -85 parts on them (85ns). I used -120's and they seem fine-- access from the Yamaha chip is slow. The E28F008SC's erase and program a lot faster externally and I had a bunch. You could also just reprogram the original part. The chips are TSOP's, so you will need a programmer and equipment that can handle that. Be careful with the chip orientation. Intel wisely (hahah) decided to have two dots in locations that can easily be interpreted as pin 1. The LARGER dot is the pin one location. (Putting the chip on backwards will probably kill the flash, but seems to have no ill effect to the motherboard-- did that once on accident.)
I also considered making a 'region switchable' hack... There are vias for virtually all the necessary signals off the Flash chips (and a couple easy traces going to the Yamaha) that could be tapped in to without too much trouble, but region switching alone seemed sort-of low value. A bank-switching board to hold pre-programmed flash images (like in a larger 8Mbit part) to make changing carts instantaneous would be neat, but installation would be a pain for most home/hobby types. So, I left that for another day unless other people think it's a great idea...
At any rate, that's all the info you need to region-change SPI motherboards. It's super simple as long as you have the tools to deal with the parts and the SMT rework skills.
-Clay