If I were directing (which I will be, someday), I would take a page from directors I respect and allow people to petition me for more time, or to at least demonstrate that their performance, while going over time, is going to be worth it. Hell, I've had to email directors to say "My track goes over the limit by 10 seconds, am I going to be penalized?" and the answer has usually been along the lines of no, you're fine. But that's assuming the director is a person who's been doing this long enough and has good control over their masquerade to where allowing people to go over time (technically) is something they feel comfortable doing.
Seriously, a 1:30 performance is pretty average and, normally, would be within most masquerades' rules as-is. It's not egregiously long. But compared to the base framework at your con, I don't know how easily you'd be able to petition for an allowance to go over by 30 seconds or whatever. If you do, don't disparage the potential crap level of other entries even if you're fairly certain there'll be some interminably long crap entries. Just explain that it's a performance you can't edit down any further than that, and use as much positive information and detail as you can. If they like the idea and like you, they might be lenient. I couldn't say for sure. Good luck, either way.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com