I remember doing this a couple of years ago. You have to use user objects and use fine grain password control that you apply to a group which contains all the users (MACs). This allows you to use the same username and password - downside is you then have a bunch of users with the same name and password, so you need to be doubly sure they can’t perform an actual login to a computer.
What about doing it another way? There are a couple of options. What about using a different RADIUS server for that SSID, I’m sure you could find a free one that does what you need. Or, use iPSK (identity PSK), either with the Meraki authentication (if you’ve APs that support it, and not too many devices), or again with a RADIUS server (NPS doesn’t work well here either). Worst case resort to a plain PSK on the SSID.