A macromodel would be done with some combination
of real devices and standard SPICE/Spectre elements.
You can find many examples of macromodels out there.
Also veriloga op amp models, which could perhaps be
easier to work with if you need to model things that
are out of the reach of poly-sources etc. The customer
would probably want some documentation showing how
the "macromodel" behavior lines up with the schematic
based simulation (and they'd really even more rather
have hardware results, although an internal little circuit
block may not be very characterizable with bench gear).
Sharing anything with a customer ought to be under a
nondisclosure agreement and then you'd monitor that
customer's activity for signs they breached it, and sue.
Maybe make more money that way than an outright IP
sale. With the right lawyer and the right luck, anyway.