Sorry for the delay in getting back on this one. I've been busy elsewhere. I suspect that we are talking on cross purposes here. Rule 14 ( below) is from the SFA Cup Rules. This sets out players eligible to play in a match, and does say a nominated substitute keeper must be on the team sheet. To answer Dick Dastardly, it could be an outfield player, but he can only be substituted for the playing goalkeeper, as a keeper. Rather than do that, bring back Kenny Arthur!!
If a game is in play, and no substitute keeper can come on if all substitutes have been played or both the original and sub keeper have come off, an outfield player can indeed be nominated as goalkeeper. Oldco had to do this in a game at Parkhead when Roberts went in goal. If I've misinterpreted, happy to hold up my hand.