Hi bmiracle and welcome !
This forum can get addicting in a sense but if you use the info positively and have patience there is almost no way the results won't some improvement.
To my knowledge there is no backdoor number circulating for USAA.
Your chances for approval are going to be based on an EQ score only with the high 600's being the consensus as the minimum.
USAA , through dicussions here, also seems to take into account how many products you use with them, i.e insurance,checking,savings,CD, as well as length of time as a member.
They will give instant approval online at times but if you don't recieve one don't be discouraged it's not an outright denial.
There CSR's are some of the best and they will gladly transfer you to the supervisor instead of a backdoor.
Unless someone else here knows of a backdoor but I have never seen one posted.
Good luck and remember , you've already been approved in a sense as a member so the battle is half over,