trademarked(?) names in ad campaign

I am working on an ad campaign that compares fashionable clothing to a stylish new medical product. Our photography displays a fashion model, with the articles of clothing identified by designer name, and the punchline of "medical product by Client Name."

We are trying to get permission from the designers for the use of their names, which we presume are trademarked. My question is twofold: if a designer declines, is it legal to use a fictitious name to identify the clothing? Also, how necessary is it to get permission - although we probably do not qualify as fair use, the market that will be seeing the ads is vastly different from the fashion market.

Please let me know if I need to clarify anything. Thanks!

I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.

#2 of people seeing it is very likely.

#1- fictitous names are used all the time

But I would attempt to get clearance first.