Does anyone know of a way to quantitatively express the amount of vibronic-coupling that occurs in a given molecule? I know that vibronic-coupling allows some transitions to occur that are traditionally Laporte [parity] forbidden; and I know how all this works qualitatively. I imagine this has something to do with UV-vis spec. Perhaps there are some "epsilon" values that are typical of these types of excitations? This is not homework, but extracurricular (curiosity killed the cat!). Any insight would be appreciated.

Have you looked in to whether this can be done using a quantitative NMR method?

No I haven't, that seems like a likely candidate. I'll look it up now that I have a weekend ahead of me, thanks.