OK, steam power isn't going in the game. I accept that much.
I still want to use magma to power other machines apart from the furnaces. A fairly obvious method presents itself.
Air pressure.
Every air tile has a pressure, let's say 4/7 at sea level. Hot air tiles will give up some of their pressure to the tile above, though they never give up the last unit of pressure because it would be very strange for vacuum bubbles to form in my magma smelters.
The exceptions are if the tile above has 7/7 pressure already, or it's got a solid floor. Then the hot air tile will look for the lowest pressure air tile among its horizontal neighbors, and if possible, will equalize pressure with it, transferring the number of units of pressure that gets it closest to having the same pressure as the recipient.
Equalizing pressure can happen with vertical transfer too, but for simplicity's sake I think it might be best to ignore it unless the pressure differential is very large.
If the number of units so transferred is greater than say 2, this transfer counts for "wind". The number of units transferred minus 2 is the strength of the wind.
Items can be blown away by the wind. Realistically you'd want to account for their surface area, but since that's not modeled in the game, assign weight-windows to each of the wind-strengths, eg. strength 1 wind moves objects weighing 1 or less, strength 2 moves weights 2-10, strength 3 10-50, etc.
Windmills use this system to figure how much power they're receiving, so build one in a cave on top of some magma and hook it to your rollers for speedy minecart action.
This wouldn't involve adding any new technology to the game, so I figure this lets me use geothermal energy without violating the 1450 tech cutoff.