Maine and your too long post>
IMO, yes you could prepare circle (engrave on your room for example) and allow item or even waiting spell to take effect on those (otherwise it would fail since no item can draw a circle without a specific effect), so I read "at the time of the casting" related to "affect everything within".
Another reason why I think it can't work (as for room and all other parameters, except wards which are explicit counterexamples, and the aegis which is a breakthrough not respecting the rules), is the limit of the arcane connection: you must be aware of a target to affect it.
When you cast, for example, a ReME spell on a castle, to make everyone asleep at moon duration and after you have left, a novice called Sebastian enter the castle, you were not aware of his presence, thus he can't be affected. And the only way to achieve it is to say, that it does only affect what you aware of, that mean "at the time of the casting", since you are not necesseraly aware of what will come after.
I think those special things are fine and provide a nice supernatural feeling. ("the castle where everyone entering fall asleep"). However, when it comes to circle, I'm more cautious : if someone is affected at the time he enters the circle, is on his boundary. If the circle is a physical boundary, all future people affected will be affected at the edge. if this is some kind of "petrification" effect, i find it really bad to have statues on the edge of the circle... without even considering that they will effectively broke the circle, freeing everyone .
But if you think the circle is a metaphysical boundary, I could read that everyone entering the circle break the effect: they weren't part of it. There is a myth (I don't remember the name) where one enter a circle, and the circle is broken at that time, freeing every one who has been kept prisoner for years, isn't it?
In front of the problems a general saying "that could work" creates in regard to particuliar effect, I tend to stick with "limit of the arcane connection + description of target". It's easier for the players, which otherwise have to guess what you will say.
But I don't deny that some of those effects are really cool!
IMS this is exactly the kind of spells you can have with experimentation discovery or modified effect, for example.