Thanks, guys - I may be in the market, but there have been a few developments since I last checked in here. Here's what I know so far:

The p/n is MD146946. This apparently is the only ECU that made it into the 3.0 Gen 1 - there is another model that is sort-of compatible electronically but does not have the same connectors, which I believe begins MD175xxx.

There are known issues with the Rubycon electrolytic capacitors leaking corrosive gunk onto the board.

The 3.0 engine has proven more durable and more popular than Mitsu (intended? thought?) and so there is a shortage of rebuildable cores for this part. My local dealer can order a new one from Mitsu for $1300. I haven't tried eBay but there seems to be one in a boneyard around here for a little less than that.

AVPro ECM in Ft Myers Beach FL can rebuild them for $160 unless they are totally fried. At present he does not have any cores in stock and hasn't seen any for a few months, but Nick (the guy I spoke with) seemed extremely knowledgeable.
He immediately identified the burned out part (a Darlington transistor array, one of four labelled IC107-IC110), and correctly told me that it was probably one of the middle ones. He also said the likely cause of the failure was - get this - improper winding insulation on the stepper motor which controls low-speed idle. Sure enough, I had to replace that part (with a new Mitsu part, cost about $200) last summer! He said that the insulation on the magnet core wire is too low-temp and eventually fails, shorting everything out, all the way back to the ECU, and he STRONGLY recommends disconnecting the idle controller and just using the screw.

He also said (this may be self-serving, but I kinda tend to believe him on the principle that whatever can screw up, will) that putting a boneyard piece into your truck might well cause it to blow, either because of the above issue (duh) or because the electrolytics could haved leaked causing damage to the board that was not there the last time the donor truck ran. For his price (see below) it seems prudent to get the thing rebuilt and checked.

Indeed, when I looked carefully at my board, I could see some corrosion around the base of C107, the larger of the two big electrolytic caps.

His price for the rebuild includes repairing any damage to the multilayer board (mine was singed a little, not too bad) changing the leaky Rubycon caps, and replacing any other bad components. They give a lifetime warranty and, as he said, 'I never want to see your board again.' I was impressed and sent him mine overnight - I should have it back here Thursday and will report fully. Anybody else out there have experience with them? As I said, I've done a bit of circuit design and this guy seemed on the up-and-up, and quite knowledgeable.