It took quite a lot of effort to persuade all decision makers to make this happen, but here it is: A new Debian buildd is being hosted at Karlsruhe Institute of Technology, to support the s390(x) ports. Its name is zemlinsky. So we've got some redundancy now and despite them being some sort of fringe architectures, they're looking pretty good. s390x is currently bootstrapped in the archive and it's progressing pretty quick. This new fast builder is one of the reasons why the slope is so steep.
Pointing people at the Debian Machine Usage Policies (DMUP) is pretty helpful to get a consent, with relation to network usage and acceptable use of the machines themselves. In this case the hardest part was drafting a user agreement that allows other non-university persons to log into the box, which is crucial to have it maintained by the Debian System Administrators.
Thanks to all the people at IPD Reussner, Steinbuch Centre for Computing and BelWü who helped me getting this done.