Any chance of an intelligent response?
No.
That's in answer to your original question, by the way:
is an accurate readout of height agl available to ATC at the big boy airports?
I'll leave it to you to decide whether it constitutes an intelligent response.
To explain, ATC see height above sea level, not height above ground level. But then any self-respecting airfield controller is going to know the height AMSL of their airfield and the surrounding area, so as LB says, it's easy enough to do the calculation if necessary.