Geronimoo implemented Rennaissance! Yay!
I tried some simulations using Flag Bearer. The conventional wisdom so far, is that getting the flag is great, but if you're the first person to get the flag then you're at a disadvantage. In the context of a kingdom with only flag bearer, this appears to be mostly correct.
I used a modified version of WinderingWinder's BMU strategy. The buy rules are as follows:
-Buy Province if $18 in deck
-Buy Duchy if <= 4 Provinces in supply
-Buy Estate if <= 2 Provinces in supply
-Buy Flag Bearer if opponent has Flag
-Buy Gold
-Buy Duchy if <= 6 provinces in supply
-Buy first Flag Bearer
-Silver
This beats an ordinary BMU strategy 97%/2%. However, there is also a simple counter strategy, which is the same except that it never buys the first Flag Bearer. This strategy beats Flag Bearer 90%/7%. It also seems that if either player stops buying Flag Bearers before the pile is out, then that player loses.
Is it never correct to buy the first Flag Bearer? Well I tried a few things, and it seems that if you buy the first flag bearer after turn 10, this gives you a very modest edge. We're talking a 50%/42% win percentage (regardless of who's first player ETA: I think first player was randomized).
Using this pair of strategies, I played around with priorities. It seems that both players should prioritize getting the flag over getting Duchies, and then the win percentage got reduced to 46%/42%. It does not help to further prioritize the first Flag Bearer. Basically, you buy the first Flag Bearer during the greening phase if you hit precisely $4, and you always take the Flag from your opponent unless you're buying a Province instead. I looked at some sample games and it seems that you often get to keep the flag for several turns because your opponent doesn't want to pass on province.
I also tried a strategy that opens Flag Bearer, and tries to snag an extra one so that it keeps the flag when the pile is out. This helps, but still worse than not opening Flag Bearer.
Conclusions? Contesting Flag is super good. The first Flag Bearer is not so good, but can be a tossup even in absence of support.