I'd say it's not a problem; I've done it before.  I just wouldn't make them too specific or unusual.  In other words, something like jeans and a shirt or a type of dress would be no problem.  But a roleplay costume, a fetish outfit (with exceptions of fetish providers), or something she might not own is better saved for a second or further appointment.

If you like dresses or lingerie go ahead and ask.  If you are looking for a specific style or color you should be prepared to bring it with you as a gift.

Remember that unless the gal is working from her home or a permanent in call location she is traveling with a limited wardrobe.

If she has a web site,wardrobe requests maybe discussed. Be prepared to pay for the clothes if she doesn't already own what you want her to wear. Sometimes a lady will wear an outfit that she had on, in her ad pictures. Always remember to ask questions politely and be respectful.
Good luck

This may be something she accomodative to, even to first timers.  She may even ask you in the phone calls before meeting if you have any specific requests in this regard...if so, let you interests be known at that time.  Good luck!

I usually ask to keep a pair of pantys.

Re: if I could accomadate I will

If I'm going on overnights or a weekend or multi hour dates I always accomadate my gentlemen. If I have what their requesting for my hourly dates I will but can't promise
Jay