It would be useful to have an XML-RPC method that takes in a bug ID and returns a hash with two entries: 'dependson' and 'blocked'. These would each be a list (possibly empty) of bug IDs that the bug depends on and blocks. This list would encompass the "full" tree, not just the immediate list of bugs.
We should do this for both the RPC and web interface. It will be done as a virtual fields (dependson_deep and blockson_deep for example). Change would be done in Bugzilla::Bugs so we can get the values for both places.