Has the author of Dominion ever settled the rule in this forum or elsewhere?
Although I doubt Donald X has ruled on this specific issue, one thing he has said repeatedly is that he considers Dominion to have a general, implied rule common to most games -- if you aren't told you can do something, you can't do it. It's not necessary for cards to explicitly tell you not to do things.
You are correct that neither the rules nor clarifications (I guess?) say you cannot look at the next card before deciding. However, this doesn't mean there's an ambiguity or confusion. What's more important is that the card doesn't say you *can* look at the next card, therefore you can't.
I believe there's a rule somewhere in the book that tells you you are not allowed to look at cards in your current deck. Unless this rule is explicitly over-ruled by a card, you have to follow that.