pre-release uploads to -proposed are accepted but cannot be published

In bug 930217, I made it possible to accept staging uploads to -proposed pre-release, with the aim of copying them into the release pocket once they'd built everywhere (and possibly once they'd passed some other tests). This worked fine as far as it went. Unfortunately, the publisher refuses to publish these uploads:

2012-04-05 14:03:51 DEBUG Publishing The Precise Pangolin-PROPOSED
2012-04-05 14:03:51 DEBUG Attempting to publish pending sources.
2012-04-05 14:03:51 ERROR Tried to publish gtk+3.0 3.4.0-0ubuntu4 in precise (2365347) into a non-release pocket on unstable series Precise, skipping
2012-04-05 14:03:51 DEBUG Attempting to publish pending binaries for amd64
2012-04-05 14:03:51 ERROR Tried to publish libgtk-3-doc 3.4.0-0ubuntu4 in precise amd64 (25316231) into a non-release pocket on unstable series Precise, skipping
2012-04-05 14:03:51 ERROR Tried to publish libgtk-3-common 3.4.0-0ubuntu4 in precise amd64 (25316225) into a non-release pocket on unstable series Precise, skipping
[...]

We need to publish these in order to be able to do any useful tests on them.