EDIT: 9/13/11 Tab completion hack now works anywhere in your sentence, as well as with the /kick and /ban commands. eg. /kick @mrchess
Synchtube now has tab completion for chat names when you prefix with an @ symbol.
For example, if I wanted to write a message to "mrchess", instead of typing the full name I could type:
@m then hit [TAB] and it will automatically write @mrchess
Hitting TAB repeatedly will cycle through all the names that began with your initial value, meaning if there was a second person named Mary in my room, by repeatedly pressing tab it will cycle through "mrchess" and "Mary" values.
Give it a shot in the chat!