Front End Asset Upload to Custom Element Type not working
I looked into this extensively and the one thing I found is that if your element type uses a custom content table, the save procedure will be done properly but when Craft needs to display the asset in the index, inside an asset selection modal, or an asset field, it simply won't look in the right place and unless you copy your asset's locale and title to the Craft content table from your custom content table, it won't show up.

I've made @Brad aware of this but they haven't fix it yet, at least not in the last release that I can tell.