dave wrote:
looking for a mechanical method to attach sheet metal to thick steel
without welding, drilling, or tapping. preferably leaving behind
tamper-proof (or 'nearly so') heads. what's a guy ask for when he wants
a self-drilling, self-threading machine bolts that have heads on them
that snap off after a certain torque is reached?

alternately, are there bolts made (just like above) except they'd have
TWO heads 'stacked' in an hourglass-like config, so that I'd have to CUT
their uppper heads off with my cutoff wheel, manually? (which'd be
perfectly fine with me)

alternately, what's the biggest diameter pop-rivet that can be 'manually
popped'? I mean like for which a tool that's "fairly readily available"
new -or- used?

Break-off head screws certainly exist: they're in the RS Components
catalogue (& doubtless others, too).
Picture a round-head bolt, with a cone attached pointy-end to the centre
of the round head. At the base of the cone is a hex. You put them in
with a wrench, & snap the cone off from the roundhead part.

Possibly not the best for you, though: looks like you'd have to drill &
tap all those holes first (PITA...)