This would be a pretty easy feature to add, but it doesn't exist at the moment. First of all, I'd need to know a bit more specifically what you mean by "block with a certain quantum number" because usually a block has a set of quantum numbers, one for each IQIndex. All IQTensor blocks have the same total quantum number (what I call the "div", "divergence", or "flux" of the IQTensor). But if you just mean one particular block of an IQIndex, then yes this could be added.
(As a technical note, what would be needed to add this would be another method similar to getElt in itdata/qdense.h but which returns the pointer to the beginning of a certain block, rather than a specific element within a block.)
On the other hand, I don't see how it would help do the entanglement calculation you are describing, because that is a fundamentally difficult calculation to do with regular MPS techniques. Probably the best approach for that bipartition would be to use a sampling method to obtain the second Renyi entropy.