Have you considered paying some sort of royalty to Minddesk to avoid the paywall? Negotiate some sort of contract where the Stonehearth edition is “paid for” as part of everyone’s game, so that there’s a larger pool of potential modders to keep the game going years and years after launch. Minddesk gets its money and a potentially larger userbase that’s familiar with its controls, Stonehearth gets its content.
Alternatively, you could also sell a copy of Qubicle’s Stonehearth edition as “DLC” for Stonehearth on the Steam page. It won’t be as likely to end up in every modder’s hands, but it’d still be very visible and plenty of people would pick it up just for the sake of completion.
EDIT: Apparently someone is already making a free Stonehearth modelling/modding tool. I suppose that makes a royalty system less attractive for Radiant if there’s already going to be a decent open source alternative.