This might be one of those cases where you have to find a positive response (as in the doe got pregnant) which could conclusively prove that it could happen. In maths we called this proof by contradiction *is a geek!*.
I have come across a good few people who say it has happened to them (well, not them, their bunnies :lol, however, whether or not that is enough proof is debateable.
there were a few threads a long time back where some people did say it had happened then later retracted that and apologised saying actually they had put tehir bunnys together there were at least 2 seperate incidences of this .
I had hamsters that bred through the bars of a cage when I was young, god knows how... So I wouldn't rule out the possibility!!
very strange? it must have been the right time(every 4 days) else the female would have attacked him and his bits through the bars rather than push her bum towards him... and the males usually have such small bits too? id never heard aa hamster do this before.