Is there any sort of way to make LAME start at the IDLE priority when encoding from the command line? It always brings my system to its knees unless I change the process priotity to "idle". Maybe this is something that could be programmed into it otherwise?

Use the "start" command.

start /low lame.exe

Yes, I know of this.

Sorry, I should have made this clearer. Is there any way to do this from a within a program that only gives you access to the command line stuff AFTER lame.exe....that is, Exact Audio Copy or Monkey's Audio, etc. where you can only enter in things like "--alt-preset standard" -- you can't enter in a full command line.

Maybe some sort of dummy EXE file that just passes the command line to the real lame.exe, starting it with start /low lame.exe?

You just need to write a little batch file, lame.bat, which contains:
start /low lame.exe %*
and put it in your lame directory. Then just select this to use instead of lame.exe in eac or whatever.
edit1: hmm this isn't working. Give me 5 minutes
edit2: right, i would suppose this to be some sort of problem in the way that these programs are calling the external compressers. In eac in the eac options extraction tab you can set extraction and compression priority to idle. Also in monkeys audio you can set the application priority to idle in the general settings. There is a similar option in razorlame.

Those options don't seem to control LAME when it is called externally, they just control the programs themselves (EAC, Monkey's, etc).