I can't test at the moment, but its possible that slots actually can be defined with a return value - however, when used as a slot, how do you want to get the value?
Since you are not calling the slot directly, but only invoke its call (as in emit a signal), and not by the object invoking emitting the signal - so what good would it be for you if the slot is retuning a value - you can't get to that value any way (in the object that emitted the signal)!
Or in other words, you can get the return value only there where the slot is really called - namely in the MetaObject but there you probably wont know what to do with the return value...

Try defining a slot that returns a value and see if the connection works.
If it does, you are still stuck with my question - when and how do you want to get the return value from the slot?