This is a great question and an interesting idea, but the best approach would be something based on a custom post type or another plugin altogether.
All of the query logic in the
[mla_gallery] shortcode comes from the WordPress WP_Query class, which only searches the "posts" table, taxonomy and metadata tables.
The Media Library Assistant is just that; an assistant that builds on all the media library functionality in the WordPress core. In particular, the "Add Media" functions are handled by WordPress itself. The work required to trick the core code into handling library entries with no associated file would be risky at best.
There is a Links Manager built in to WordPress, but it looks like a deprecated relic of an earlier age.
I will add this to my list of good ideas, but I can't say when I might have time to pursue it. I appreciate the suggestion and your interest in the plugin; thanks!