Paul Folbrecht wrote:
this had me wondering if ATC is even making any
distinction between IFR/non-IFR GPS!.)

The short answer is "probably not". Like I said, controllers are not
pilots, and I suspect most of them have no idea about the regulatory issues
surrounding GPS certifications (nor should they).

There is one official way you communicate to ATC what navigational
capabilities your aircraft has, that that's the equipment suffix on your
type code. File /U, and they'll give you clearances you can execute with
VOR receivers. File /A, and they'll expect you to be able to identify DME
fixes. File /G, and they'll expect you to be able to go direct to any
en-route fix and fly GPS approaches.

On the other hand, if you file /U and put "VFR GPS on board", you're
leaving it to them to guess what you want, since "VFR GPS on board" has no
official meaning. The most common guess seems to be "treat me as if I had
filed /G", so they do. It turns out that this is indeed what most people
want, so it works out and everybody's happy. You seem to be wanting
something different, but I'm not sure what it is.