I have searched but cannot find anyone trying to run STRAIGHT 87 octane regular gasoline in their multifuel deuce.
Yes it's been tried, and yes it will run. It doesn't like it though. I can't find it either right now, but A, gasoline was an "emergency fuel", and B, what you get at the pumps now was outside the criteria by a (probably) survivable margin, but still quite significant. Gasoline is much better blended with other things.
I am asking because the one I am about to look at is about 70 miles from me and gas is almost 40 cents a gallon cheaper than diesel.
You forgot one variable in your equation. Mileage. If I pick some numbers here for you.... If you get eight miles out of a gallon of diesel fuel and five and a half miles out of a gallon of gasoline, what is the least cost per mile? The better value?
so.
Straight gas, will it work? EGTs stay acceptable? If not, what is a safe ratio of gas-diesel to mix?
Will it work, yes. Will it save you money? Absolutely not. To break even, if diesel fuel is at $3.50 then the gasoline would have to be selling for just over two bucks. The cheapest fillup you'll get is straight diesel. Adding gasoline in any amount costs you money above and beyond the diesel fuel. Gasoline is better saved to mix in with heavier fuels to help stuff them through the fuel filters, and the rights and wrongs in that case do a much better job of averaging out.
Money aside, if you want to burn gas just to say you did and you're willing to pay for it... Up to twenty percent and you'll never know it's there. After that you kinda start to notice it but it builds up on you.... With higher concentrations it starts a little hard (not bad) and won't get out of it's own way. Not to mention not being able to drive past the next gas station.