Thanks to the message from Stan Shebs I was able to boot over serial a
IIvx 5/230 CD with the netbsd10.patched kernel, booter 1.4 and the standard
1.0 binaries.
   Having read his message I decided to detach the monitor to disable
internal video (I also have a nubus Toby frame buffer card) and the system
booted and works fine while before this I just had 12 gray bars (actually
13 with one blanked) with the newer kernels and 16 gray bars with the
kernels netbsd.CI and netbsd.071194.
   However, given it's impossible to disable the TFB card by simply
disconnecting the monitor from it, I don't know if the problem is with
internal video or simply with multiple video cards.
   Apart from this (big) problem, which is different from the well known
adb one, it works quite well. I've used it for some hours without problems.
The only rather odd thing is memory usage (in the boot message it says 2.5
over 5 meg used which is more than double that of a Sun 3) although maybe
I've seen this on the FAQ.
   For those who want to have a fast look at it, to boot multi without
crashing the file /etc/ttys must be changed: mark off ttye0 and mark on
tty00.
   One last note: I booted with the terminal over the modem port, I don't
know if the serial port works for remote logins but certainly serial boot
echo doesn't work on the serial port on all but the netbsd.CI kernels.