Yep, it would be difficult to maintain a consistent radius with that method. My assumption (I've not looked) is that you could offset the surfaces from the two

Yep, it would be difficult to maintain a consistent radius with that method. My assumption (I've not looked) is that you could offset the surfaces from the two sides (say A and B), and then calculate the intersection of an offset of A with the original B. Then do the opposite for the other side. I think that will give you two edges that will produce a consistent radius. Offsetting surfaced and calculating intersections aren't exactly easy problems to solve on their own. Fillets are hard! (Solvespace is awesome BTW! If I had the time (or the expertise) it also makes a brilliant foundation for all this)