I was travelling from Verona, and explained this to the host. I also kept in text communication with the host throughout the evening, detailing my progress in finding the car-park on the mainland, getting the tram, then the vaparetto to Rialto.
At no point did she mention what was to transpire later.
I made our rendezvous at the appointed church at 21.45 and texted her. Around 5 minutes later she showed up, and we went to the apartment which was only a couple of minutes away. So we entered the apartment no later than 21.55.
The host then went through the usual stuff, the heating, the electric, the locks, the bathroom, the windows, etc. with me. Then she mentioned the city tax of around a Euro, which I was expecting.
Then she flabbergasted me by demanding 20 Euro for "check-in after 10pm."
I though she was joking, and I said we had been standing inside the apartment for 5 minutes, and it wasn't 2200 yet. The host just cackled weirdly and flashed her phone at me which now said 22.01....
I REPEAT we had been inside the apartment for a good five minutes, and no warning of this charge had been mentioned previously in any case.
So, the moral is if you would like to spend a night or two in a Venetian garret (tiny and next-to-useless heating) and lose a "pound of flesh" in the bargain, by all means book it. At least you'll have an amusing story to tell your friends back home about the "venal Venetians!"
The only bad experience I had in two weeks travelling in Italy, I might add...