I have a Scanjet 5P, and the docs are pretty adamant about not using the card with anything other than the scanjet. Specifically, it says "The HP interface card provided with the scanner is not designed or tested for use with any equipment other than a single HP scanner. HP does not support the use of this card with SCSI devices other than HP scanners."
Of course, that may just be for their own protection, but I wouldn't get my hopes up about the card being useful for anything else. I especially wouldn't be optimistic about it working with some sort of chain of devices.