Did I have a conversation with you about this on Facebook? If not, you're the second person with a recently "inherited" drum scanner I've seen.

NOTES
You're reaching the Chinese casino. That means you have a good internet connection and you are connecting to the net.

"Security failure. The server reply is invalid." means that when you launch the utility, it goes online and "phones home" to verify your installation is valid and authorized.
One of two things is happening. Because the address you're going to is now a casino website, either the scanner company is shut down and gone or at least has changed its internet address for authorizations since your copy of the utility was released. Possibly it worked before because there was an updated copy of the software on the failed drive.

You need to locate the correct web address, possibly by finding and contacting the company to see if your software can simply be updated with the new address OR if you need to get a newer copy OR if there's a way to now run this old stuff without having to authorize it each time OR if it's an old dead horse and you're stuck.
Long shot: Try launching the utility with the internet unplugged. If you're lucky, it might reveal a way to do it without an internet connection.
I take it you don't have the user manual to look for this info? Google the company.

Nifty little feet! Your method should work just fine.
The CD drive thing IS a concern though. The disc should be clamped down pretty tight once it actually gets in, but it's going to be difficult to keep in in the tray properly on the way in and out. Then again, maybe not.
I suggest you just try with a blank or junk CD and see what happens. Maybe you'll develop a technique of keeping your finger on it just so until it's secure…?

To kill the UM-1 family trouble.

there is an installer and it can be downloaded here:
there is seperate installers/settings for freemidi vs oms
http://macos9lives.com/smforum/index.php/topic,3612.msg27363.html#msg27363

AND... Photoshop 7.1 and Illustrator 10.0.3 are available via Macintoshgarden.org (Photoshop 7.1 runs on G4's and G5's, OS9 and Tiger... without all the baggage and clutter of the Adobe CS package).

I have tested identical file tasking in both 7.1 and the Photoshop CS package on the same large image file... and Photoshop 7.1 was MUCH faster on the G4 and G5... and OS9 Photoshop 7.1 versus Photoshop CS on a G5.

Of course, it takes a while to download and build your install CDs from MacGarden, but it was entirely worth it for me.

And yes, inexpensive Firewire drives are abundant and readily available.

Due to space restrictions it would be beneficial to mount my G4 on its side as opposed to the normal vertical tower position.
As you will all know, the case sides have a gentle convex bow which prevents me from just tipping over the case on its side as it just sits on the crown and is completely unstable.
Having unscrewed the 4-off countersunk screws on the case sides they appear to be M5 thread and have found some nifty little feet on ebay which look like they will just screw in.

http://www.ebay.co.uk/itm/ARTICULATED-FOOT-Series-10-PA-20mm-Thread-M5x40/282358786958?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

I may have to shorten the threads of the feet, as I just need to keep the case crown from touching down, as they look a little long.
Anyway, my question is can anyone think of any reason why this arrangement wont work besides the difficulty of loading a CD in the tray which would be vertical?

Thanks ...Sweep