hey there
I go to a little place on top floor in Bin Sougat near the metro and they are good, just collected some jeans and all fine. Its called Aroos I think, just go in the centre part and up the escalator and its right there on the right opposite the juice bar, basically above Spinneys and slightly along, big glass windows and you can see all the abayas hanging. Very helpful, the best I could find !!! good luck !
There's a place in the corridor between Nautica and Athlete's Foot in Ibn Battuta that does it. They cut the original seam off and reattach it so you can't tell they've been altered. I have all mine done there.
<em>edited by HereComesTheSun on 03/12/2012</em>