1786 MO Ff 2 Reales - How Rare?

Yes, pretty scarce... def. moreso than the rarer assayer of 1784 (not the GRTIA error). My observation is that Krause is a bit overblown pricewise on some of the scarcer/rarer in-year assayer varieties in the Bust series (most readily noticeable for Mexico). Not many examples, and I think its even more of a specialist collector thing than, e.g., the Indian-head Lima 2R that was discussed... so it's hard to gauge demand. That said, you got this pretty cheap, I thought. The tape mark would deter some people (not much you can do to help that), but still decent...:
http://www.ebay.com/itm/MEXICO-COLO...380715954398

The one listed variety that does always seem to do pretty well (contrasted with assayer varieties and perhaps also overdates) is obvious engraving errors. Noticed, for example, that a one of the CAOLUS Chile 1/2R popped up on ebay recently and sold quickly for what it was priced at... More major/obvious, I guess. I actually think that error might be just as common as the correct version of that year (1797, I think)... but for the series, it's a nice error type.

Consider this... also Ponterio from earlier in 2011. They had up a 1784 2R GRTIA error, slightly more wear, with a much higher estimate than that 1786 FF 2R. Didn't sell, it seems, but still... compare that to what you see in Krause.
http://www.mcsearch.info/record.html?id=476659

Interesting that it's a different obv die... there couldn't have been many different die pairings for this date/assayer, I wouldn't think... And, certainly, this ebay piece is genuine. Have you found an obv match with an FM reverse?

See, the one thing - bringing up traits like relative die states, different obverses... By and large, I just don't think the community that generally collects Spanish/Spanish colonial coinage cares all that much about such things... The level of sophistication (pedanticism?) involved in, say, Overton bust halves, VAMs, State Coppers, or similar just isn't there. Maybe if enough people join you, there will be.

The one Spanish coinage sector where the early American collecting crowd has jumped in and really tried to get into the particulars (die pairings, etc.)... is actually the fake stuff - namely, the Kleeberg-type contemporary counterfeit 2R.

I will say this - I stumbled on a thread about VAMs on the PCGS boards yesterday while looking up something else... 30x or bigger blown up pics of these barely noticeable, inconsequential die flaws... I just shake my head. There's a fine line, I think, between sophisticated collecting... and giant geek.