My only thought is that if you can use some kind of already existing system
(ACRL or ARL reporting?  Marshall Breeding?) I think you will have better
luck than requesting voluntary participation by either libraries or vendors.

Certainly, lots of libraries will voluntarily contribute these kind of data
to a joint effort, but will they keep it updated?

Won't we need to contact that library and make sure they're still using it
"just in case"?

(Although a database that tracks use of various systems over time and not
just each year would also be tremendously useful.)

The problem with vendor reporting is that it will be hard to get full
disclosure in a timely way across all vendors.

But even with that problem, it still makes more sense to have a limited list
of vendors who absolutely know who their customers are doing the reporting
than to trying to get every library in the country or the world to report
voluntarily.

Marshall Breeding uses (AFAIK) press releases from vendors to track system
changes and to report losses and wins.  So that's vendor reporting, albeit
indirect reporting.  Maybe he also sends a questionnaire to vendors each
year?

I think it would be ideal if ALA or ACRL would do this kind of data
collection.  I know it's hard enough to do your ACRL and ARL stats every
year but they are valuable and most institutions do them.  A few more
questions that are fairly easy to answer would provide a very rich data
source, although of course, adding even a single question has significant
impact on data processing, survey creation, etc. etc. etc.