Mmmm.. there may be a slightly simpler way to do this.

The SMC ethernet chip has a spare TTL-output pin (as well as a spare input), which could be wired to pin-8 of the ds1821 temperature sensor chip. Then the pull-down to GND, and subsequent pull-up to Vdd could be accomplished automatically by Hijack.

Just a simple pin-lift + jumper wire. And it could be left in place permanently.

I think I'll "kill" another temperature sensor here and try that mod on it..