Speaking as one of those recent entrants to the world of piano roll
scanning, I have been giving this database matter a bit of thought.
It really would add value to the existing corpus of scanned roll files
if we could agree on a set of metadata keywords _and_ find a way to
make it widely available and editable by contributors.

Warren Trachtman's IAMMP web site seems to me to have the most complete
database as far as inclusiveness goes, and I routinely consult it in
order to decide whether to scan a roll in my possession, so as to avoid
duplicated effort.  But even that database doesn't show the lesser MIDI
metadata such as music category, length, tempo, etc.

I have little familiarity with the e-roll format, but I can see its
value from an archival perspective.  My own scanning operation produces
bi-color CIS files which provide similar archival usefulness.  Perhaps
there might be some way to accommodate both in any widely-accepted
database.

In any event, I also feel strongly that any attempt to reach a consensus
on metadata standards will likely founder unless the majority adopt it
immediately and vigorously.  Toward that end, perhaps a crowd-sourcing
approach along the lines of Wikipedia (presumably locked-down with a
list of authorized editors) might be the best way to do it, so that no
one person bears a major maintenance burden.

Marshall Jose