Hi Jordan, we are using something similar that is an extension of what you have:

basically as each user logs in, they add a record to a table and RLOCK() it.
at the same time you can delete any unlocked records (crashes etc), you can also check number of users at this point and prevent access.

here is a simple example

* always do this in the same datasession
SET DATASESSION TO 1
SELECT ULOCKS
SCAN FOR RLOCK()
    * if you can lock it, no-one else is using it
    DELETE
ENDSCAN
APPEND BLANK
=RLOCK()