This is the point where I'd suggest splitting impromptus. Keep regular "impromptus" for people who are there at the time and want their maps played while they're available, but use the planned ones (would need a different name) to play through the list with.
Sounds pretty much just like gamedays, but taking submissions through the bot rather than the forum. Which could work I guess - though I know a lot of people only use the forum to schedule tests. Perhaps greater education about our systems is what's needed.