Just got back and tested a bit further.
1. The actual message nvramcui printed when hung was: initscr(): Unable to create SP exited with status 8
2. May consider switching SeaBIOS to stable (1.12.1) and try again, as some SeaBIOS patches (like the one that enables multiple floppy disks) currently do not apply on master. I'm interested in SeaBIOS' support status for NVMe on recent versions, but I've read somewhere that the support was initially only enabled for QEMU, and I'm not sure about the current versions.