You're right, neither of those options is cheap. There was (is?) a guy selling these, rebuilt, on eBay for about $100. I got one and hot-water tested it. Can't remember why, but I sent it back and was graciously refunded. In any case, this is really a low-tech, unreliable device at best.

As Planetman noted, and my own experience with my AAV being completely disconnected has shown, the actual, practical warm-up period is not very long. Yet, should I have to leave the car outside for a protracted period in winter, I would like this option.

So, rooting around in my parts drawers I've found two items, both under $10, that I think I can combine to make a reasonable operational facsimile. One is a readily available 12V programmable digital thermostatic switch (Marlin P Jones P/N 32764-MP), the other an eBay 12V solenoid air/water valve. The T-stat device has a temperature probe that I can affix to the block and the valve would be inserted between the two existing air hoses. Using the temp switch to control the valve and playing around with its off/on temperature set points, I think I'll be able to get a good approximation of the performance of the original, lacking only the ramping function.

Will be playing around with this and report back with diagram and photos if it works. Meantime, keep those alternative suggestions coming. Always more than one way to skin a cat.