I only have a tip which might make it a little less annoying. If you are able to select the desired item, but it only takes you a while to get there, you can assign an F-key to quickly use that item without having to use the backpack. When selecting an item in the backpack, just press any key from F2 to, I think, F12, and you will assign the item to it.
Apart from that, I could only think of a rather inconvenient workaround in the form of an AutoHotkey script. It could be set up so that pressing a certain key, a specified other key will only be pressed a single time instead of being held down. Thus, if you normally use WASD or the arrow keys for menu selection, it could be made so that you can use the numpad for the inventory, and this way you should be able to use it normally. I could give that a try, if this problem won't get solved otherwise.
[edit]
I made a little script which binds the u, h, j and k keys to the arrow keys as long as the script is active. The difference is that there is a tiny delay of 10 miliseconds between each keypress, which may be enough to make the inventory useable. Here it is, in the form of a standalone exe-file:
http://www.mediafire.com/?j1dmkjg2to4 or as the script itself, which requires AutoHotkey to be installed:
http://www.mediafire.com/?ngqtyzjj4mm It's mainly based on
Jagge's script for using the mousewheel in Outcast.
Hopefully that helps a bit until the actual problem is solved.
[edit²]
I just noticed that it's probably more comfortable to be able to, for example, hold down the Alt key to make all keypresses done while holding it slower. So, you could just control the game like normal and then only hold down Alt while in the inventory. If the script works at all, that is.
