If we assume that the restriction takes precedence (which is not currently part of Dominion rules)
Isn't it assumed that something like this is in place though? Otherwise how do we know whether Moat or Militia gets priority?
Hmmm... that's an interesting example/question, but I still think it's different than 2 effects, one which says "you must do X" and the other which says "you can't do X". With Moat, it specifically calls out the other effect and says to ignore it. It's more like Coppersmith in that it changes the effects of playing another card. This is different than simply having a restriction that says you can't do something.
Though, this does make me realize that you could probably re-word such a restriction so that it would work, though it would be messy: "Until the end of turn, card instructions that tell you to play another card have no effect."