How to make a persistent counter?

Dear FreeRADIUS users: I am trying to implement some load balancing for our wireless network. Each user needs to be placed in a different VLAN to distribute the use of each one. If I had a persistent counter that I could access, I could use the rlm_expr module to take the modulus of the counter and use that to generate the next candidate VLAN. Does anyone have any idea about how that could be accomplished? Ken