I don't think the 150 can support SCSI Disc Mode.. It says nothing in the documentation I have and according to a few sites as well.

http://lowendmac.com/tf/2k1129.html

http://docs.info.apple.com/article.html?artnum=16918-en

I would address one of your questions in the original post and test with my PowerBook G3 Series Rev 2 *if* I had the proper SCSI cable and *if* I had a decent sized HDD that had not failed like my PDQ's old 40GB samsung.

*Edit

Nevermind, an answer to the G3 PB topic in found at a developer.apple.com link here:

http://68kmla.org/forums/viewtopic.php?t=4388