Will the interaction of two proteins vary across different tissues?
Your example of an interaction mediated by a scaffold protein is certainly one way of controlling protein interactions. This doesn't just happen in different tissues either, it is also used to fine-control the interaction of certain proteins within one cell, the most notable example for this are the kinases in the MAP pathway (see for example this paper).

Another possibility I can see is that A and B can interact, but only if (at least) one of them is modified, e.g. by phosphorylation - so the actual interaction happens with A-p and B. In a case like this it's not unlikely that the protein that modifies A is only expressed (or active) in certain tissues, which then mean A(-p) and B can only interact in these tissues.