hi robb, sorry i didn't see your post earlier. i hope this reaches you in time to be useful.
i'm a north american woman who has lived & ridden in iran (though i didn't tour there by bike). i rode mostly in yazd, a smaller city. while rare for women to ride bikes, it is not illegal or risky. sadly, social pressure keeps them out of the saddle.
'chadors' are NOT a legal requirement, blacky is mistaken. modest dress, however, IS ('hejab' in persian & arabic). i wore a one piece headscarf that many students & office workers wear; it is a single tailored piece, fuss-free & doesn't need retying. i can't remember what it's called in farsi; you will make friends very quickly & i suggest asking them about it.
i usually wore something very similar to the woman on the right:
http://www.pbs.org/adventuredivas/iran/media/show1/iran_7.htmltrenchcoat, headscarf, jeans. though my sleeves were only an inch or two away from my wrists, not as short as hers.
2 tips:
-before riding, i safety pinned my trenchcoat to my pants' knees, just so it would stay in place & not fly up in the wind.
-a woman wearing a regular scarf could use a barrette or bobby pin to secure it, just so she doesn't have to worry about it. or wear a baseball ball cap on top of her scarf. this is totally fine & not disrespectful.
locals & authorities understand that foreigners are new to hejab, & will appreciate/sympathize with your efforts. as always, take your cues from those around you.
be prepared for everyone to be surprised to see a chick on a bike. it's pretty funny for most. good for you guys for showing that biking is for everybody!
regards & happy trails,