Singlespeed - zip tie okay?

Saturday morning obligations prevent me from getting to my race time for some of the earlier season cross races. I've heard talk about zip ties used to make a geared bike a single speed for one off races. Is that something that OBRA and race promoters would allow? I'd love to be able to get to a later race, but I don't have the $$ to drop for another rig that is a single speed. If I could zip tie, that would be great.

Thanks!