Have you tried using hot rocks a la stein brewing?
Manticle, no I haven't. MIght have a look at it though! Prior to the advent of widespread, cheaply available thermometers and measuring equipment I think not only would the making of beer have been more intuitive but it would have relied on a much wider variety of techniques. I've read about a couple -
1) Finnish and Norwegian style, setting up a mesh of juniper branches over a fermentation pot, putting the cracked barley malt on top of that, and then performing the mash and sparge at once by just pouring hot water through it a couple of times.
2) Simply setting the pot at a set distance from the fire and letting it gradually warm to the correct temperature.
3) Heating the water up to boiling point, taking the pot off the fire, waiting for a bit and then adding the barley malt.
Might look up hot rocks. Now I wonder if the following method might yield worthwhile results:
4) Warming the water up to 50 degrees or so - easy to judge because this is about the temp of the hot water from the tap. Add cracked barley malt, put pot on stove over extremely gentle heat so that the temp of the mash rises only gradually. Stir occasionally to encourage circulation and make sure nothing gets stuck. Keep heating while the temp of the mash rises incrementally; the important thing would be to get it slowly through the 66-71 degree range I think, to encourage the fermentable sugars to come out. (This is a reverse variation of no.3 - not sure whether either method is better, but it would probably work best with large quantities, as they store and maintain heat more reliably than small quantities).
When I made a (very small amount of) Sumerian ale it felt very simple and intuitive, like making a soup. Never bothered with instruments.