But it seems to me that what you describe is not so much that the PVC
absorbs energy ("loss") but instead: that it unavoidably de-tunes the
antenna because of its high dielectric constant

PVC is L-O-S-S-Y at 2.4 GHz. The amount varies according to manufacturer. CPVC
has the least loss of a lossy lot.

OK?:-)

73,
Chip N1IR