The Update Channel Selector Extension provides UI to change the update channel. I suggest that this or something similar could be added to the SeaMonkey preference window (Advanced->Software Installation). See the attached screenshot.

I'm not sure we should have this for all users by default, and additionally the channel is not a pref but a setting file in the build itself, so we need to deactivate/remove it when we have no write permission on that file (which is the common case at least on the common Windows platform). I'd vote for this being in debugQA or such, easily installable but not not a default part of stable releases.

Hmm. Good point especially on Vista with its enhanced security model. In this case I think this should be a WONTFIX as there is an extension to do this anyway.

WONTFIX for the time being as it depends on write access to the SeaMonkey installation directory.