Although not web based, one option I can think of would be :
1) have a project to act as library,
2) one action which allows to re migrate the assets to/from a given project .
(could be a reworked version of this example: https://bitbucket.org/ftrack/ftrack-recipes/pull-requests/4)
This could be extended to keep reference (as metadata or custom attribute) on where the asset originally comes from (as project when publishing back).
3) a pyside based browser (so can be used as widget in maya, nuke and any other qt based application) to act as library browser.
Although requires some work, seems doable.