If we haven't set up a way for the play booster to work with MKM yet, because it's a brand new way of formatting, then there's no way to make it work for anything fantasy currently. Even when it's set up to work with real sets, we'd have to have a dev that is willing to code something to allow fantasy or other/older sets to work with it.
The other custom stuff you were talking about in general exist because someone has spent the time to code support for said custom thing to work. Custom things don't just "work". Support for such things have to be designed that will allow users to be able to make their custom things. We have very limited man-power these days and as far as I'm aware there isn't any dev that would be looking into allowing the new play booster to work with older or custom sets, nor anyone with the time to create a tutorial of how to change the engine to do so.
Currently, our limited time is being spent on adding real products and things to Forge which leaves very little time for fantasy/custom stuff. That's not to say it will never happen, just that someone has to want to do it.