Intro:
As some of you know the 315-5313 was used in a bunch of SEGA arcade boards, besides the MD, all (or almost) of them more powerfull than the console.
The truth is these boards used the VDP and some auxiliary grafics chips and pals aiding them...
Main:
The System C2 was a surprise for me because it uses the VDP, but RGB isn't connected to anything! (WTF?) instead it uses 8 pins for pixel data (pin120-pin127(?) ), these ones get into a colourize chip (LOL) wasting the VDP crappy CRAM.
What's the point?
The point is that you can build a CRAM yourself with 12-bit (or more), and with this you can choose from a total of 4K colours (Like the GG)
Cool what else?
Now, you all know the MD pallete is:
----BBB-GGG-RRR-
but with 12bit it becomes:
----BBBBGGGGRRRR, the thing now is that the GameGear probably uses this pallete format (it was the easiest way, while maintaining backward compatibility with SMS), so this will probably means that GG games will work on MD with full colour!
Yeah, and'bout those bits and SPA/B?
SPA/B, probably means Sprite Plane A/B
I think it's to differentiate Sprite pixels from Plane pixels, but you need to activate it by using the bit 4 of register 0C (I think)
Why do I need it?
Well you now have 64 colours out of 4096, but with SPA/B you can use a different pallete for sprites, this gives 128 (without sh/hi and changing colours on the fly) colours on screen (-8 for 0 indexes)
Sumarizing you can have plane A/B using a CRAM and sprites using another one
So those pins, the pixel data, what do i do with them?
Well I don't know how to use them yet, but they just tell the index of the colour they use (just like the tiles).
Ok then I have those pixels, how do I "colourize" them?
You need to implement 2*64 banks of 12bit-CRAM, Shadow and highlight mode in a FPGA/ASIC/Whatever then output Digital RGB (if youre using 12bit CRAM then 6bit/colour is more than enough) DAC the colour signals and feed the RGB encoder of your MD.
Credit for this goes to Charles MacDonald for it's docs, you will find them at http://cgfm2.emuviews.com
Don't forget this is post all speculation, To prove the truthness (!) of this I need to test that register and SPA/B, look out for the GG pallete format, test those 8 "unused pins" and discover which pins they are...
IF anyone is doing it, I will probably do if it's true I just need to know how to capture CRAM DMA's so I can copy them to the new cram...
Don't forget that SPA/B is tied high with a resistor, so it is better to take it out before setting the bit to 1
Sega always think about many things 
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