Righto, not even British coins.
OP, the coins may end up in the bottom of the plenum. If so one of two things will happen. Either the coins will jam the temperature blend door or the door will just clear the coins and you will have no problem. The second scenario is the most likely. I have never had to remove coins to free up a mechanism. Pens, yes, lots. Crucifixes, yes. Peanuts and other food items, yes.
You'll probably be OK, don't worry about it.
(The coins can't get to the fan because the AC evaporator forms a very effective filter and the fan [blower] is on the other side of it.)
Uh, last time I checked coins are not magnetic.

Don't think this will help much.