With default WM6 and 6.1, you can set press and hold of right (red) hardware key to lock phone:
Start > Settings > System (tab) > Long Press End Key
If you must have it on softkey, then maybe KeySwop by Vicott Wong of Vicsoft would help. I used keyswop to set my right softkey to \Windows\Backlight.exe, because of the bright Florida sun making my screen unreadable when outside. Now, without even looking at screen, I can press end (red) hardware button and know I'm at today screen home. Then a tap in lower right screen area, and finally holding the right navigational edge of directional control pad until screen is readable or at max. Best of all, can do it 1 handedly, with thumb of same hand holding phone.
Keyswop is freeware, here:
http://mobile-sg.com/software/?p=KeySwop&platform=ppc
Hope this helps, MrG