AFAIK almost all header settings and information sent with Arcsoft in a picture message can be edited, as it can on hacked dumbphones. However, are we sure that Sprint is blocking access to their MMS servers on the grounds of authentication information sent with the pictures, and not simply blocking MMS access based on the account number; Sprint has all of your devices registered by ESN, and knows that you have PDA's, so is it not entirely possible that you are being blocked by esn number, rather than some Arcsoft Settings?
This would mean that there is entirely no way to circumvent the system. However, I know that some people with ppc 6700s cloned their devices, and therefore would be registered according to sprint as having different phones than they actually own. Someone with a dumbphone ESN on their apache needs to try to send an MMS, and if he can, you are being ESN blocked. If he can not, the system can be worked around, as they are blocking you because of some data that arcsoft sends.