Well... I used it, before I went all Diablo-happy. I knew I'd go back at some point, but wanted to try out the new improved mce.

The good news: It's not necessary with Diablo, if you're lucky. (I was, the General wasn't, and I think a couple of others may have reported experience one way or the other...) If all you want is softpoweroff, I think that's largely attainable by editing /etc/mce/mce.ini. (I must warn you that there's a warning in mce.ini similar to the one in powerlaunch about making it impossible to shut down... but if you're careful with either program, it's pretty straightforward.)

The bad news: I am currently without a powerlaunch install to give you any guidance from. (Yes, most all of my accurate reliable guidance comes from looking at existing files, not from memory.) Moreover, I have never messed with powerlaunch on Diablo (final release or pseuDiablo), so I don't know what's different. I don't have any (pseu)Diablo install at the moment. (I begin to despise Patriot SD cards, though I'm not sure the card is defective, or I just crash/freeze + hard reboot too often.) So I can't even give you particularly great help with the mce.ini approach either.

But, here's from memory:

• Get root, and edit /etc/systemui/systemui.xml
  This isn't at all necessary, but it's helpful to uncomment the whole section there, or at least the softpoweroff and reboot entries. This adds those items to the normal menu.
• As root, edit /etc/mce/mce.ini
  This is pretty clearly documented with comments. You'll want to find the short powerkey press option, and change it to softpoweroff. You may want to change any of the other powerkey press options as well; take heed the warning, of course. Finally, you may want to change the softpoweroff behavior, too.
• Reboot to make everything take effect.
  This is unnecessary and really a bad idea, but it's easier than trying to remember:
  
  Code:

  for STUFF in `remember stuff NOW` ; do
          /etc/init.d/$STUFF restart
  done

  to restart mce and systemui.

IIRC, my actual working config involved double-press power for softpoweroff, short-press for menu, and long-press for poweroff; not going to offline mode, and restoring offline status when leaving. (I think there should be another option to keep online mode when a BT headset is in use, but there isn't, and I haven't gotten to filing a feature request in Bugzilla yet...)

If the systemui menu fails to come up sometimes, you can do a long- or medium-press (not sure) on the powerkey, then try it again.