Well, I spoke too soon
Turns out one of my nodes was able to download the 9.1 image successfully through option 7, but not the other. Really wierd. So I have poked around and in the /cfcard directory (via systemshell) there is an env directory that holds an env file. There is one particular bootarg that seems to get set when invoking Option 7:
setenv bootarg.install.ramdisksize "512M"
I don't know enough about the sim to know how this comes into play, but I'm guessing this is used to create an area where the new image can be downloaded from my http server. The reason I think so is, using HFS, I can see the node get about 65% through downloading the image when it fails due to the /mnt filesystem being full. That is right at 512MBs of data having been transferred when it barfs. The actual 91_q_image.tgz is 777MB.
I tried editing this to create a larger RAM disk, but the value was changed back somewhere in the sequence to 512M, as it failed again at the exact same point.
My /cfcard filesystem has 1.4GBs free and the 9.1 image is only 374MBs on the node where it successfully downloaded. I've looked for differences in the various filesystems (again via systemshell) and there just isn't anything significant ... all very minor. BTW, i tried an 8.3.2 image and got the same results. It also exceeded 512M ... as it is 527M,
Anyone have any ideas as to what may be going on here? Really odd that my other node had no issue with Option 7.