Unfortunately, I think that many schools know extremely little about TBI. I have worked in a school for 8 years (I'm a neuropsychologist) and though we have a large variety of behavioral/emotional/learning/developmental/attentional disabilities represented at varying degrees of severity in the school, we've had EXACTLY ONE TBI student, and his TBI was a birth injury, so it was a VERY different story.
it is not crazy to wait until the school year actually starts to do the eval (we do this at school regularly- all the districts and charters where I live are no longer required to do summer evals- the "clock stops" the day after school ends and restarts the day before it begins again). Also, a GOOD evaluation will involved much more than testing- especially with a student who has suffered a TBI (and the issues are- IME, far more individual than most ADHD students I"ve tested, who usually have at least SOME issues in common). ONce your DD has "settled" a bit into the routines of school, observing her (in class, in social situations, etc) will be extremely helpful. Also, in many cases, supplemental service providers (i.e. speech, OT/PT) may not be available to test/observe until the year is officialy under way.
HOWEVER, I'd definately request a 504 now. while the school likely has NOT done much before with TBI students, they have probably done one for other medical issues (diabetes, etc.) and with your consultant's help, this can be a good starting place as the full evaluation is started (plus, even if they "started" today, in most places they have 60 calendar days to finish). A 504 for your daughter may start off by focusing on the more "physical" manifestations of TBI. Will she need the option to go to the nurse to nap/rest at certain times of day, or as needed? is she sensitive to light and need sun glasses in doors? Do her attention issues benefit from preferential seating, etc.? Does she have balance or other issues that would indicate the need for an adult to walk near her on steps or on the way out of the building in an emergency (i.e. fire drill?) Does she have difficulty with directions/getting lost (i.e. would she need an escort to the bathroom or other places in the school where the teachers might expect she'd be fine going alone?
Also, you can talk to the principal about other options, such as starting with a shortened day (i.e building up her stamina) if needed-- some times this can mean coming in for a half day, or coming later in the day if the morning is hard for her. Sometimes this means they give some amount of home bound instruction as well.
hope that helps!