I doubt, the devs will ever let you select an item, and let you scroll it off screen while it still has the focus. As you can see when you scroll, you are essentially pressing the Up/Down arrow keys, the selection moves not the screen/window. If you find yourself getting annoying, you can always look into separate Clipboard Manager app. From my experience:
1. Keyboard Maestro(good, a lot of other things; perhaps too many)
2. iClipboard (good, but you have to use the mouse/pointer to select, so I don't like it. You clearly don't mind it, just might be what you need...)
3. Clipboard Centre (simple, basic)
4. Copy 'em Paste (haven't tried it but heard good things)
I believe there are demo for all these apps. I would like to say I'm not in anyway related to these apps or devs, just my personal experience. If you do use a separate manager then don't forget to turn off LB's clipboard manager; otherwise your might see lag in copying large files.
Nonetheless, it still would be wise to shoot the devs an email ( launchbar(at)obdev.at ), they might include your feature to solve your conundrum.