Hi Derek,
There are a couple of approaches you could consider. You're quite right in that giving general users access to a DEV environment is not ideal.
Assuming that for whatever reason, mastering data in STG is not currently possible, you could nevertheless consider creating a snapshot project in STG for the purposes of running your forms entry. The snapshot will then sit outside of enforcement rules, enabling your form entry to work. Also, it's acceptable to calculate a snapshot in real-time, meaning that you wouldn't have to contend with the warm times of a normal project in the STG/PRD environments.
There are other possibilities, depending on your specific usage scenario. However, the best approach is largely dependent on the profiles of the users that may wish to run this analysis, and the frequency/response time/level of TBMA interaction that is required/acceptable in this scenario. In short, if you want them to be able to function with a degree of independence, they should be competent and trusted users anyway. If they are not pretty experienced with Apptio, I would imagine that you would want to oversee their activity to ensure that not only is data entered correctly, but also that the results delivered are interpreted correctly.
Hope this helps!
Regards, Ian.