This Mastermind tip comes from Nathanael at Formidable Support. Thank you Nathanael!
Maybe we've never noticed this before, but it was pointed out in the Formidable Slack community that the source code for hidden fields does not display on the form's Customize HTML page. In business applications, hidden fields are not only used for their ability to hide values, but can also be used to hold custom or data- attributes. Not having the source code available to add these attributes is, quite frankly, a bummer.
The solution is actually quite simple and we're a little embarrassed we didn't think of it first. But thanks to Nathanael from Formidable's Great Support Team, here's the solution:
We recommend using any standard field type that suits your need and then hiding it with CSS. Fields that are not Hidden Fields can still be hidden from view by using the Formidable CSS class frm_hidden.
There it is! Simple and yet elegant.