Thank you for the information and your question. Unfortunately, if you are an equity partner, you are the employer in a legal sense. So, even though you are, for tax purposes an "employee" of the firm, the legal theory that the courts would use to make their determination is the same as that used when denying partners and business owners unemployment benefits, even though they are also employees.
I am guessing that you have read the code and the definition of "employee" and "employer" but if you haven't, you can see it here: http://www.dol.gov/vets/usc/vpl/usc38.htm#4303
You might also want to read a couple of items written by attorneys for the Reserve Officer
Association that confirm the conclusion that partners (as long as they have equity in the practice) are not covered under USERRA. You can see those at:
Of course if you are not an equity partner, then USERRA would cover you as long as you meet the other requirements.
Please let me know if you need any clarification. I would be glad to assist you further if I can.