On the topic of crediting... Is it possible to use Codeshare flights instead of prime codes when booking segments?

What rules apply here? Should I assume married segment logic applies?

So, for example on a AA operated leg which has a BA Codeshare, I could presumably only select the BA* Codeshare flight no if I'm connecting to/from another BA leg?

I ask because after the recent changes to BA's program, some airlines earn considerably more than others even when Codeshares.