I have used the sensors from earlier iMacs as well as from Macbook Pros. The Macbook Pro aluminum models are the ones I usually use. There are several of them, I usually use the ones that are under each fan. They have 2 leads just like the iMac which I extend to reach the drive. Then i attach the sensor with double-sided tape (same stuff we use to adhere glass on iPads and unibody MacBooks).
The sensors from earlier iMacs (if they are the 2 lead type) also work. Some splicing is usually required.
I have done it this way since the very first drive replacement I had to do on one of these iMacs. I don't think that any resistor or jumper is a good idea at all. Temperatures in a computer need to be monitored and the fans adjusted continuously for everything to work at peak efficiency. I don't want to leave fan control up to my clients to monitor. Nor do I want their iMacs running noisily because their fans have to be run at high speed all the time.
I'm perfectly willing to hack and modify things when it makes sense, (my Hackintosh is over- clocked to 4.7GHz, and I install Mt Lion and Mavericks on unsupported Macs all the time) but in this case it doesn't make sense.
Macbook Pro A1150 right fan ambient temperature sensor. Splice one of these (black to black and color to color) to your old sensor cable and stick it to the hard drive. The hard drive fan will function just as it did with the original drive.
People have over-thought this problem.
My thinking was:
I can't use the internal sensor so I need an external one.
I have all these other mac parts.
Maybe the sensor from another Mac will work.
Lets try a Macbook Pro sensor.
Splice the wires and try it out.
Works fine, all temps check out.