Yes, there is a limit of 5 icons in subradial. Also feats with subradials are fixed (you can't change subradial items in game). So we would have to make every possible 5-element combination of spells for given class and then make radial feats out of them (for 10 splls there are 252 5-element combinations if I calculated correctly). That's why I can't imagine how this would work.