Perhaps if bug assignees cannot be given the 'canconfirm' bit, it might be possible to add all proxy maintainers to an appropriate group? It looks a bit odd from the outside, if a [proxy] maintainer cannot actually verify whether something is indeed a bug or not, and seek clarification from upstream/submitter if needed.
*** This bug has been marked as a duplicate of bug 668040 ***
*** This bug has been marked as a duplicate of bug 660848 ***