A simpler extension may be to allow the class to be specified in the validate method e.g.

A a = new A();
v.validate(a, Relaxed.class, A.class);

I tried defining

@GroupSequence({Relaxed.class, A.class})
public interface RelaxedAndA {};

and validating against RelaxedAndA but that generated a "Cyclic dependency in groups definition" but it wasn't clear to me why - probably because it was anothjer redefinition of Default for A