You could rewrite portions, fix portions, do extensions, and contribute back any of these to developer. These are quite advantageous esp if it's small enough for customers to understand. One of the first systems to send the source to customers and accept good contributions into next release was Burroughs B5000: a 1961 mainframe that cost more than a house.
> You could rewrite portions, fix portions, do extensions, and contribute back any of these to developer.
Contribute back to the developer yes but it gets real iffy if they don't accept your patches that you need. Also really depends on the licensing regarding rewriting and fixing portions. Depending on the platform you're using you may need to shove that into a repository for delivery of your customized version; is that type of distribution going to be allowed?
I didn't see an explicit "here's the license all of our stuff uses" unless I missed it but a lot of these use cases could be pretty difficult IMO.
Oh, Im not talking about this license. Im talking about how to do a hypothetical license combining payment and OSS-like advantages. In terms of what you asked, you could word license as such that they could do about anything they wanted to it... from applying local mods to distributing them... so long as the users are paying customers. Optionally, getting perpetual use of each year's release if longevity is an issue.