Well I tried it again on a fresh reboot and it performed as described:
1 - with "video" module blacklisted it booted up at full brightness and the fn-keys did nothing.
2 - did a "modprobe video" from the root console and fn keys worked 6 levels of brightness in the correct direction.
It could be this action in combination with the 27-odd ideas I tried from this board and others (gotta luv Linux!)...
Prior to this change it seemed that the brightness would work but was being over-ridden by software. Hitting the fn-up would cause the screen to flash bright then go dim, the flash would be the "right" brightness, then it would be immediately changed to the wrong brightness. Somehow this eliminates the 2nd change.
Now I have to find a way to add this to the end of the boot-up routine so it works automatically.