True, sorry, I didn't look closely enough to realize that tag_summary is denormalized change_tag.
However, this doesn't deal with the problem that [[Special:Tags]] will get cluttered with this approach. It might work for a "mobileedit" tag, but valid_tag cannot grow arbitrarily.
True, but that's already a problem with the tag system. EN Wikipedia's Special:tags is full of things with description "This tag is inactive.". In the long run we will probably have to find some way of managing having a very long list of tags.
Another problem though is people might want to track what edits are mobile (or whatever else) but they may not want each one to be shown in RC as mobile edit (since it adds clutter). Perhaps using the currently unused ct_params to be able to make certain tags hidden (be able to filter by them, but not show up in the line in RC) would be a solution.
-bawolff