I could not find any potentiometers smaller than 5k ohms at Radio Shack, and was not willing to wait around for Digi-Key/Mouser to ship me one.
So, I put a 1-pin weatherpack connector on the wire extension I installed yesterday, then installed a 1-pin weatherpack jumper with a 220 ohm resistor on it. Now, I can very quickly switch between stock and added resistance. Hopefully, the 220 ohm resistor will be small enough to be acceptable by the engine computer. If not, I might as well go to plan B above, because I just don't see that it's possible to get any sort of meaningful increase with anything smaller than 220 ohms.
I've added the below table in an attempt to illustrate the # increase in EGR valve opening with the added resistance. "Stock" is what the engine computer would get if it commanded the EGR valve open at a certain percentage, without the added resistance in-line with the position sensor. With the feedback from the valve position sensor with the added in-line resistance, the EGR valve would open to the "modified" percentage value.
% EGR Opening, Stock and with 220 ohm resistor added
| Stock | Modified |
| 0.0 | 0.0 |
| 5.0 | 5.4 |
| 10.0 | 10.8 |
| 15.0 | 16.2 |
| 20.0 | 21.6 |
| 25.0 | 27.0 |
| 30.0 | 32.5 |
| 35.0 | 37.9 |
| 40.0 | 43.3 |
| 45.0 | 48.7 |
| 50.0 | 54.1 |
| 55.0 | 59.5 |
| 60.0 | 64.9 |
| 65.0 | 70.3 |
| 70.0 | 75.7 |
| 75.0 | 81.1 |
| 80.0 | 86.6 |
| 85.0 | 92.0 |
| 90.0 | 97.4 |
| 95.0 | 100.0 |
| 100.0 | 100.0 |