I would like to run 9legacy on OpenBSD as well. Unfortunately, I'm still learning and even though finding and ideally contributing solutions to such bugs is a distant goal of mine, my only current option is to run on 9front.

In the wiki (https://wiki.9front.org/openbsd-vmm) it says: 
"Since VMM doesn’t provide any display functionality, 9front’s console has to be redirected to virtual serial port by adding console=0 to plan9.ini. It makes sense to also use display=none while at it.

Note that even though you might not have a chance to modify plan9.ini before booting, stopping boot process with an early hit of a key, typing an additional configuration in, and then boot<ENTER> is ought to work."

Unfortunately, the only way I know to start a VM under OpenBSD is to set up the vm in /etc/vm.conf, enable and start vmd via rcctl and then connect via SSH or vmctl console. 

Could someone possibly help me out and explain how I can edit the plan9.ini during the boot process, please?

Thank you very much in advance!