Here's some context: I want to implement a Kinko's style self-serve credit system for the big tools at our maker space. Members scan their RFID cards to sign in to the laser cutter, CNC machine, 3d printers etc. and credits are deducted from the members account per job. The part of the scenario that's relevant to CiviCRM is where to store the members credit amount and how to add to it. All the deductions happen without CiviCRM.
The overall idea I had, is for a custom contribution page to read the users current credit amount value from the database, add it to the contribution value, and put the result back in the users credit amount field.
Finally, my questions! If I use Custom Data to store the users credit amount, is it possible to set the input type to None? This shouldn't ever be a value that gets manipulated without a contribution, or deduction. Am I better off creating a table manually? If you were in my shoes, would you use Custom Data for this? I'd like to keep it in the system if I can, so others can readily use it.