In the literature on political opportunity structures (POS), it is commonly held that federalism means more access points. More access points mean more opportunities.
It does not suffice, however, only to look at the number of access points. Leaving aside the fact that some movements may not be acceptable to potential access points, more potential access points mean a greater probability of findings an ally. However, more potential allies also means that the influence of a single ally is more restricted. Therefore, it is not at all clear that the opportunity (as in getting influence on policy-making, for example) is greater in federalist systems.
In federal systems, the entry might be easier, but the impact might be limited. If an organization gets access in a more centralized system, the impact is at once more significant. For this reason, I suggest that we should expect no significant differences overall.
Perhaps there is another mechanism that favours federalism, perhaps it is the inclusive political culture? In this case we should focus on the relevant variable. Perhaps it is acceptable to concentrate on the degree of federalism as a measure of political culture (inclusiveness), but the argument would need to be a bit different.
Didier Ruedin 