So I'm told that ASL can be converted to written form via something called "gloss" - http://www.lifeprint.com/asl101/topics/gloss.htm
I've heard of FF readers learning ASL by treating it just like any other language, writing down sentences in gloss and doing fill in the blanks with that.
The only added step I'd imagine might be handy is that each gloss word would benefit from a pic or an animation to make sure you actually learn the sign for it. So if you have a sentence in gloss like:
YESTERDAY PRO-1 INDEX-[at] WORK HAPPEN SOMEONE! MAN CL:1-"walked_past_quickly" I NEVER SEE PRO-3 BEFORE
You might want additional cards showing the sign for MAN and quizzing you on the proper gloss for that sign, and another card showing "MAN" and asking for the sign for it.