Can I boot a different plan 9 kernel without changing SD card contents?

Look up '/dev/reboot'. Though note that this requires you use s9pi4 file rather than 9pi4. 's9pi4' is the 'a.out' while '9pi4' is a flat image for raspi bootloader.

It is possible to actuate /dev/reboot with a boot parameter. Example: reboot!/sys/src/9/bcm64/s9pi4!tls