I wouldn't expect the rules to say anything about this; the card text is intended to be the rules. There's also nothing in the FAQ about it.
Angel Feather reads:
When you attempt a roll of any kind, you can call out a number from 0 to 8. Use that number instead of rolling the dice.
To me, that fairly clearly implies that you choose the number before you roll - you use the number instead of rolling. If you could use it after you roll, I'd expect it to say something more like "After you attempt a roll of any kind, you may choose a number from 0 to 8, and use that number instead of the number you rolled."
Rabbit's Foot says:
Once during your turn, you may reroll one die. You must keep the second roll.
Once you've rolled, it's too late to use a number instead of the roll. Once you've gone pat that and rerolled a die, you're even more clearly past that point. It sounds like you're right - you can't use these things together.