The MDM feature you are referring to is already available under the “Passcode Policy” payload. The key, specifically, is “Maximum auto-lock time” and the value you’re interested in is “Infinite”.

Worth noting is that Apple MDM does not actually let you specify the auto-lock time on a device. Rather, it lets you specify a maximum value only, then permitting the user to specify their own value that meets your requirements. If Apple expands these capabilities in the future, we will be happy to implement them.

Finally, there is one way to disable auto lock and that is in conjunction with single app lock. If you are planning on restricting the user to just one app, you can use a “Single App Lock” profile, select the app, and check the box next to “Disable auto lock”.