I think that's a good idea, actually. Sometimes there are posts, that would fit into multiple channels. Of course, that feature would be ripe for abuse, e.g. would make spamming much easier. So it should be limited to users which have been here for a while, and don't want to ruin their reputation...
Implementation-wise it would work by changing the sub-field from a many-to-one into a many-to-many relation. This would require some rewrite, but it seems possible to implement... But I don't know the source of poal, so I can't say.
(post is archived)