One way to do this would be to have a base object class, and use dynamic_cast<>(). You can do this even if you're wrapping a set of classes that don't already have common base class. Say you have classes Foo and Bar, and a class Baz which is derived from Foo and Bar. You then derive classes

class Obj {...};

class MyFoo : public Foo, virtual public Obj {...};
class MyBar : public Bar, virtual public Obj {...};

class MyBaz : public Baz, virtual public Obj {...};
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You have to wrap the constructors, but otherwise these objects act just like your original classes. Be sure to give Obj a virtual destructor.

Whenever you pass a blessed object to Perl, static_cast<>() it to Obj before casting to void.

void *ptr = (void*) static_cast<Obj*>(foo);
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Reverse the cast when you get a method call

Foo* foo = dynamic_cast<MyFoo*>((Obj*) ptr);
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The blessing into the correct class should even work automatically, but check the XSUB code to be sure.