MBA to MPA..hmmmm...interesting...too bad you didn't decide on accounting a while back..would not have needed to get an MBA.
As for McCombs MPA, here is the student profile: Profile of MPA Students, Department of Accounting, McCombs School of Business, The University of Texas at Austin
As you can see, the avg GMAT is 671. I'm sure that your resume/qualifications far exceed that of the avg MPA student who has no work experience so you will probably get some leeway on the GMAT score. My guess is that if you are in the 600s you are probably fine. The lowest they accepted in the 2008 class was a 540.
Interestingly enough, the website says you must have a 3.0+ GPA to gain admisison. However, in the 2008 class noone has lower than a 3.24 (which is higher than my ugrad GPA) and the avg GPA is a 3.72...wow! Once again, I'm sure you could receive leeway here too but if you are expecting a lot of leeway on the GPA and GMAT then you are probably pressing your luck because work experience is not as important in an MPA program as it is in an MBA program.
And...if you are interested in getting your PHD in accounting you can dismiss everything I wrote and just look here: Admissions, Doctoral (PhD) Program, Department of Accounting, McCombs School of Business, The University of Texas at Austin
which says they generally get 650 and often get 700+. It also says you should have 1 to 2 yrs of accounting experience.