Jim, it is my understanding that the LDS church did not finance prop 8. Individual Mormons did. That is different than the church itself doing the financing or if the religious leaders imposed some sort of sanction to those members that did not finance. Of course, if either of those can be proven....
As for taxing a church, the congress (or State legislators) could certainly do so, by changing the laws. How far do you think they (legislators) would get, if they did? <- Yeah. Rhetorical question, as we all know the answer.