I am now trying to install from a RHEL10 s390x DVD. This seems to be going OK, it certainly boots and runs code from the initrd. This suggests that the problem is with the Debian trixie s390x ISO, and not with virt-manager.
There is a secondary problem with the 'virt-*' support, though. The first attempt to start the DVD failed saying that network 'default' is not active, and when I run 'virsh net-start default' I get a message that the network can't be started because the ethernet is already active. To get round this I click in the top right corner and turn 'wired' off (network-manager, I think) and then 'virsh net-start default' succeeds and I can turn 'wired' on again.
