Flexible Inversion?

I have occasionally been asked if there is merit to inverting the meaning of a 3♠ cuebid and a serious 3NT call when hearts are trumps. I have thought this through a bit more and have a thought to share on this topic.

If hearts are set as trumps, after a natural spade call, then I believe that cuebidding spades naturally at the normal, "cheap" level of 3♠ is more important than the serious cue, largely because the existence of that spade cue is critical to partner deciding how serious he is.

As an easy example, consider a very basic auction. 1♠-P-2♥-P-3♥-P-3♠. If 3♠ is just a spade cue, then Opener can better decide how serious he is by reference to his spade holding. If 3♠ is "serious," then Responder is deciding how serious he is without reference to whether Responder does or does not have a spade card. It seems to me that the better course is for Responder to first tell that critical fact before the partnership decides seriousness.

Another example. 1♥-P-2minor-P-2♠-P-3♥. Here, again, it seems that Opener should define his spade suit quality as his primary function at this point, with Responder then showing seriousness in light of that fact. Arguably, however, this might be different, where Opener might want to show serious interest with a 3♠ call, to "get under" the ability for Responder to now cue the critical spade honor for him. I'm still undecided here.

So, perhaps the inversion should occur (if you like that idea) unless 3♠ is Responder's first chance to cue a spade suit opened by partner?