I absolutely get where you're coming from! I'm sure every provider on this board has gotten their fair share of, "Are you available now?" emails or calls lol.
I'm starting to think that it comes down to a fundamental misunderstanding about the way many of us work. These guys, a lot of them have 9-5 type jobs that they clock in and clock out of. Between certain hours of the day, they're simply "at work". And I think they're under the impression that our job works the same way lol. It's kind of a sexy thought, actually... Lounging around in our hotel rooms perfectly made up and dressed to the 9's, on call and waiting for our SuperEscort pager to go off lol.
But to answer your question about whether it's a good idea to ask for an additional donation for last minute dates, I think it depends on what the core of your issue with them is. Is it that you ARE willing to do them, but want to be compensated for the hassle? Or is it that you CAN'T do them and want to discourage clients from even asking?
If it's the first thing, I say go for it. It might turn off some clients, but you don't really like doing it in the first place, so who cares? If it's the second thing, maybe you can try mentioning it specifically in your ads and on your website first. Something along the lines of, "All dates must be booked at least X hours/days in advance." might help weed some of these last minute guys out, if you put it explicitly on your ads and contact form. I'm sure there will still be people who can't be bothered reading anything but your phone number asking to meet in 30 minutes, but at least then you can be justified in ignoring them lol.