"I didn't mention fork offset or trail because I want the option to use a sus fork, and I believe those figures are fairly standardized."
They are only standard on standard neck specs. You need to pick a fork, or at least go with a fork your builder likes, then just get a rigid fork that will have similar specs. Stability is going to vary more in relation to the trail figure, than the head angle. Also where the bags hang in relation to stearing geometry makes a big difference, and how they are loaded and with how much gear, how high, etc... Personally I would probably not go for a suspension, Sakkit for instance has not really figured that out from what I read in his literature, it can certainly be done but getting something out of it is tough. There are a number of world tourist who do it. One in particular has a website and use a Koga. This requires a suspension capable front rack. I tend to not see loaded touring going hand in hand with the kind of stuff that compresses a shock, and if vide is the issure there are a lot of other ways of dealing with it.
"I discovered that when I put front panniers on my folding Bike Friday"
There are a lot of factors at work. I like the way a loaded bike stears, but the whole thing has to be designed to do it, it is about getting ballance in trail of the bike geometry and the load geometry.
Your chainstay can be pretty long without hurting anything. Someone might come along and say it will hurt something, and the next day we will have a pationate thread on the advantages of cycling the Big Dummy in rural South America.