I wonder if having her hang out in the younger 2-3 yo room at daycare will provide the motivation you need. At this age, they figure out really quick which room is for 'babies' and which is for the 'big girls'... Does the preschool have a younger class? She may have also have over heard you saying that as long as she is not, she can't go... Which may be fueling her decision to fight you on it...?
I bought my daughter a toy that she really really wanted, and kept it in the bathroom up where she could see it but not play with it... And told her, if you want the toy you have to use the potty for a week. Rewards for toilet use don't tend to work for 2-3 yo children, but at her age, she's old enough to understand it. And, be prepared to take the toy back to the store (with her seeing the reward get returned) if the goal is not met...
I do understand though, that if you're up against trying to get her into daycare, that you might not be able wait "until she's ready"...
Good luck!