FYI: I found a work-around that will suffice for at least simple scripts. It relies on the "On Error:" behavior "Stop executing on database with error" option.
Using a two script approach, script 1 serves to "pre-qualify" the database. I created a simple pass/fail query: "select count(*) from ConfigDistrict" - that's a table guaranteed to exist in each of the databases I'm looking to reach, and equally guaranteed NOT to exist in any of the databases I don't want to reach.
If script 1 fails (i.e. table not found), then script 2 isn't run for that database, but MultiScript forges on to the next database. If script 1 succeeds, then script 2 is invoked for that database.
Suitable only for the simplest, single-script tasks, but it's a start. I do still have to build a Distribution List of "all databases on all servers" every time I've got a task to run (since that list is a moving target), but at least it's a fairly swift "select all, Add>" process.
It would still be nice if an upcoming release gave Distribution Lists some kind of selection criteria or filtering option!