It looks like programs/taskmgr/taskmgr used to let you edit debug channels for any process, but now that function seems broken; when I right-click on a process and select 'edit debug channels', I get column headers Debug Channel Fixme Err Warn Trace but nothing under them. Is this supposed to work?
In the meantime, I'll make do with http://wiki.winehq.org/Debug_trace_toggle_key which is kind of what I was looking for anyway.