If you have the patience and knowledge to create and package metadata, then, yes, it can be shared. Cridgit is your man for this as he has written voluminous tutorials on the subject and has posted quite a few metadata files on ShareCG (for example for "Elanor" by Mirabilis http://www.sharecg.com/v/58502/gallery/9/Plug-in/Elanor-Metadata).

As far as I know, there is no main "clearing house" and I guess, if anyone is doing this at all (other than Cridgit!) they are uploading it in various places (I don't think there is any at Renderosity). The problem is, of course, the sheer number of products and the knowledge needed to package it for sharing, not to mention the time and trouble. Frankly, I don't think many people are going to be doing it.

I create my own partial metadata for all the products I use regularly that don't already have it. I create enough for it to work on my system (and at this minimal level it is not too difficult or time consuming). However, a lot more work is required to create metadata for sharing, and none of what I have done can be shared because I don't create "products" so there is no "support" file, as I don't find it that useful for my own use and it takes more time.

As for vendors, it is (I presume) now a requirement for products distributed by DAZ, and I think all vendors creating for Genesis really should be creating metadata for these products, but I think outside of DAZ the process is not widely understood and the usefulness of metadata not widely appreciated—and metadata used only by a minority of the market, so vendors don't have everyone clamouring for it.