Hi, I'm following a similar path with thinking about and trying to write the code for converting a GTB sys1 to OPP + MPF. If I can get this achieved I thought I would write my own expanded ruleset for the game.
Are you aware of the plank boards for OPP - there are some restrictions with the newer ST processors. I think the plank only fits on one side due to different pin arrangements on the ST. I believe the new processor will be able to support serial leds which may change your thinking on lamps as it would allow a string of led to be run from just one pin.
Not sure how you're making your printed circuit boards, but some of the gerbers I got from the OPP repository didn't seem to work when I uploaded them to SeedStudio - but that may be something I'm doing. It's a fair way down the track for me, so I put it aside until I made good progress with the code.
Good luck with the MPF programming - I'm finding it has so much flexibility that there are many ways to achieve everything and it's hard to work out which way is "best", sometimes it's hard enough just getting something to work at all. I bash away and usually make some progress there but it's a steep learning curve and takes lots of time. The MPF documentation also seems to lean more to DMD style machines with an emphasis on slides which is a little different to what I, and I think you, will be trying. This is not a criticism of MPF by the way, it's great that it's available for us and a massive effort from those creating it which i really appreciate.
Good luck and I hope you keep this thread updated as you progress. I plan to start a thread on my game when I feel I have enough to say.