My personal answer is that it wouldn't bother me if I learned of a proprietary package were developed that dropped into OpenACS 4 as a package. I would strongly encourage folks to GPL their work as a first preference, of course.
How do other people feel? My personal answer's based partly on my belief that it wouldn't be a license violation as long as no modifications to the GPL'd core and/or other packages were included. In other words, by being pragmatic I can avoid having to think about philosophical issues.
Even if one were to argue that somehow using the APM is equivalent to linking against a library there's nothing to prevent someone from giving instructions on how to insert the proper data into the database by other means...
