"So in summary, I wonder if someone has adapted a small tube amp design to allow for measurement of preamp tubes?"
Yes, it's called a 5F1 champ. I know you already seem to have ruled out an "in circuit" test methodology, but the criteria that people are most concerned with regarding preamp tubes are tone, gain & microphony - the things you have to listen for. If a preamp tube fails, it might make alot of noise but it's not going to damage anything, so testing for problems that you might in a power tube (that can blow screen resistors when it fails for example) isn't as necessary.
A voltmeter can easily tell you how the tubes differ in practical terms of voltage/current draw by measuring voltage drop accross plate & cathode resistors, hook up a signal generator and you will see amplification factor.
I seem to remember Bruce Collins stating that he took this a stage further & built a fixed bias, parallel SE champ style amp for tube testing/matching. Want to be able to AB different preamp tubes of the same type in an instant, go one further & build a fixed bias stereo champ & compare channels? If you can find a PT with a 300-0-300VAC to 330-0-330VAC B+ secondary, 5-6A 6.3VAC supply, 3A 5VAC this will let you test all the common octal power tube/5V rectifier types too (A BF Deluxe PT will work for 6V6/6L6/EL34/5881 & 5Y3/5R4/5U4/5V4/5AR4/SS etc). You can test tubes on it, play instruments through it, hell even play your I-Pod through it if you have decent speakers/hook up to hi fi speakers.