The right way to do this is with HWCAPS bits to identify whether the
hardware can run v7 binaries and for dpkg to refuse to install stuff
that won't run.
This is how it works on x86:
with HWCAPS for cmov and cmpxchg16b
However I'm not sure the arm HWcaps list:
actually covers the extra v7 instructions. You could probably use the
VFP3 CAP as a proxy on PI, but it may be better to define an actual
'has v7 ISA' bit)
So to use this mechanism would need the kernel to check for only-on-v7
instructions and set/not-set the bit. Then arrange for packages to
have needed HWCAPS set in metadata for dpkg to check.
As this data is not already present in everyone else's packages
you'd be best just adding it for the PI packages you've built.
Which of course means you could actually use any old item of metadata
'X-package-ISA:v6' and check that, refusing to install anything else,
rather than a properly general HWCAPS method. But doing it right would
be generalisable to many problems of this class (multimedia-optimised
libraries, with without/NEON etc).