I dont know if you are familiar with ATX power supplies. But a standard power connector for the various components is a 4 pin molex connector. One pin is 12V, one is 5V, and two are ground. So if Im understanding correctly, there is only one lead for each output.
So, what Im asking is, could I simply connect the pos of the fan to 12V, and the neg of the fan to 5V, without any adverse effects, to run the fan at 7V?
But as for the original topic:
Using an LM317 type device is a great idea. Its, cheap, easy, and provides a great signal. However, I have to ask, should I consider a switching regulator with accompanying circuit for signal smoothing?
Is the output voltage of an LM317 based on its input voltage? For example, If I wanted to set the output voltage to 3V, could I switch between using a 5V input and a 12V input?
Also, you mentioned heat dissipation of the LM317. So to minimize heat dissipation, I could, for example, use a 5V input for voltages less than 5V (assuming the LM317 has no voltage drop). But would the trick of using a positive voltage as ground work to also make an optional input of 8.7V (12V-3.3V) for the LM317? This would mean that, assuming I use multiple input voltages of 3.3, 5, 8.7, and 12, the maximum voltage difference the LM317 would have to provide would be 3.7V, reducing its heat dissipation.