Hi Wal
Can't say that i have seen anything like that myself. Radius turning attachment is fine if you want to machine a large radius and probably the best way to go. I modified my boring head to fit in my toolpost. large radius tools really need a fairly substantial machine(ridgidity).
If you can be bothered making toolholders you could have a look over @ Felder woodworking, I have one of there spindle tools that takes various sized carbide radius bits and these should be available as a separate item.
When making the holder make sure the insert is well supported up to the cutting edge, might have to make a holder for each different size bit.
Hope that helps
Mike