Alright, I think I figured it out.
I just purchased a Mini Sticker Transponder and just entered a bogus plate number. Once my account was created, I'm able to add/modify vehicles online, so as soon as I get to my rental car I'll just go online and update my account with the plate number.
According to the SunPass website the the “SUNPASS ONLY”, “E-PASS ONLY” and “LEEWAY ONLY” lanes can be used immediately after entering your vehicle info. Other lanes might take until the next morning to activate.
Only thing I still need to figure out is how to use the sticker without actually affixing it to the window so that I can reuse it next time. Might have to get a bit creative.