There is openmainframe project https://www.openmainframeproject.org/ ,
which I believe offers access to z/VM instances hosted by Marist
colledge.
At the openmainframeproject EU meetup, it was indicated that SUSE
joined with indication that Open Build Service might be able to use
resources hosted by Marist.
I wonder if it makes sense to reach out, and see if there are
resources available to use as porter boxes & build boxes. That way
Debian might be able to get such donated resource available on ongoing
basis and hopefully with some hw support.