I would say that you should wait for another yawn or two, just based on my experience with Spencer, she was very clock oriented and although I had to start getting her ready for bed at the 45min mark, that was only so she could wind down to fall asleep at the 1h mark on the dot...I'm thinking if he is waking at night more than just to feed that he needs a wee bit more than 30ish mins of A time in the day.
When you do end up extending A for visitors, what is the longest he has handled and still had a decent nap? I would think that would tell you what he is capable of and give a good starting point to shoot for. I wonder if the fuss before naps when A time is short is because he is UT?