Currently the SOCKS5 bytestream doesn't use relays when trying to establish a bytestream with a muc contact (we check in gabble_bytestream_socks5_initiate if the contact is a muc one and then don't add proxies to the stanza).
In practice that means than sending a file to a muc contact or any muc stream tubes will fallback to IBB if direct connections are not possible with the peer. The tube case is the most annoying one as if more applications start using muc stream tube, the XMPP servers are going to hate us.
That's because XEP-0065 used to have a bug making impossible to use a relay with a muc contact. See  if you are interested in the glorius XMPP details. But, as pointed on  that's not longer the case and an updated version of the spec now has a solution  for this problem.
We should update our implementation to make use of this updated protocol and re-enable relays with muc contacts.