Ok, here's a brief update, until I can spend a little more time on this later in the week. The new Presto stainless cooker has a vent tube that has a nut inside the canner, the thin metal of the lid has about a 1/4" hole in it. No threads in the lid, since it is thin, and the tube tightens against the nut.
The older mirro actually threads into the thicker lid of the aluminum. I can't gauge the thread now, its around 3/8" and finer than 16 tpi.
But overall, I think I can drill out the new presto, and add the older stem of the mirro. Instead of tapping the thin metal, I can easily fit a thin nut inside, if I can find one in brass or stainless. If it leaks a little steam, I think I can cut a thin gasket out of one of those silicon baking forms. They take high heat, and are food safe.
I will follow up with photos later, and any other insights are still welcome. I'll post how it turns out.
Thanks,
Brian
ETA: I see Dragracer Rob points out that the thin metal of a pressure cooker versus a true canner may allow too much fluctuation in temperature, and produce a bad product. This merits some looking into as well, although if the pot maintains an even pressure, I would think the temperature should remain within the range as well. Interestingly, it seems most canners are, in fact, thicker aluminum, rather than thin stainless.