using ping, add ons to limit membership quantities

I have the need to create multiple access levels/subscriptions but limit each one to a single subscriber. I've read in the forums that the limit isn't a feature of the plugin in its current state.

However, I was wondering if I could use the ping feature to accomplish the same thing. Here's my idea:

1. Create a custom PHP script and put it in the addon directory. The purpose of the script would be to accept a Subscription Name as an input and then mark that subscription as private.

2. Create a PING that calls that PHP script and passes the subscription name.

Here's my question(s):
A. Would that work?
B. What would the PHP script need to do to accomplish this? Is it just a matter of a database update 'set public=0 where subname=%subscriptionname%' or something like that?

Thanks for the help!