John,
For what it's worth (not much), I used to run a 1/2 wave dipole
over a metal roof. It worked. The input impedance was reasonable,
certainly not the best, but reasonable. Your loop wuold show some
directionality, I would think, but that may/may not be 'bad', and
the input impedance ought to be adjustable with the capacitor, sort
of, maybe. Also ran a full wave 80 meter loop around a building
with aluminum siding. Had pretty fair clearance all around, so didn't
really affect things much (I'm sure it did, but it worked so who
cared?). Give it a shot, see what happens, then tell us!
'Doc