Is there a way to tell Middlegen to use a List rather than a Set for a
one-to-many relationship?

Yes - change the velocity script. Remember you will have to remove the inverse="true" line as well as selecting an appropiate index column.

In my case LinkedHashMaps would be preferable and the solution is the same: customize the velocity script. Changing the collection or map type seems common enough that it should be a Middlegen relation property you can change from the UI or update in the preferences file.

To some degree this can be done reasonaly easily but a general solution requires much more effort. If my load decreases sometime soon I'll do it. Having said that help is appreciated.