Hey ShortFuse,
I've done the mod you are talking about on my Einstein. As suggested earlier, I used one of the speaker-outs as a switch-input. Takes a little work; nothing serious. Took a Marshall footswitch and wired it accordingly. Works perfectly.
That said, there are some things to consider.. It's hardly noticeable, but putting the Einstein in Mode Clean, Crunch or Mega has some sort of effect on the other channel's "compression" (for lack of a better word). Also, in order to make good use of the mode-switching, you need to set-up your Gain and Volume EXACTLY right. There will always be some difference in volume, but there is a sweet-spot that will make them all sound kind of equal in terms of volume.
These days, I don't use the mod anymore. I put my Einstein in Crunch and dial it in the way I LOVE it. On the verge of break-up, but not quite there.

I use a Fulltone OCD to give me some extra drive. Then go up and over with the Lead channel. This gives me a better sound and all the versatility I need.
If you are looking to make the Einstein a 4-channel amp, I suggest you look at amps like VH-4 or Hagen. IMHO, if you set the channel up in a way that will make all modes sound equal in volume, you will miss out on all the excellent tones this amp has to offer. Or you could just settle for "average"... in which case I doubt you would have pulled the trigger on a Diezel in the first place.
