I think I've figured out how to do what you want. Here are some steps. The following assume that you don't use the "part of a compilation" option or don't care if this option gets reset.
1. Move all of your music to the external drive. To do this:
1.a. Open iTunes preferences, Advanced panel. Make sure "copy music into itunes library" and "keep itunes library organized" are checked.
1.b. Still in iTunes preferences, change the location of your iTunes Music Library (under Advanced) to the folder on the external drive where you will eventually want to store all of your lossless files.
1.c. Choose Advanced|Consolidate library, and wait while iTunes copies all of your music to the external drive.
2. In iTunes, select all of your lossy files (i.e. the files you want on your internal drive). Press Command-I to get info about these songs and then check the "Part of a Compilation" option. Press "ok". (iTunes may take some time to complete this step.)
3. In iTunes, select all of your lossless files, and un-check the "Part of a Compilation" option.
4. Re-open iTunes preferences, and change your iTunes library location back to the place (probably on your internal drive) where you'd like to keep your lossy files. If it asks you something about copying files to be consistent with your options, choose "no".
5. Now, open finder and find the folder you specified in step 1. Open it and the "iTunes Library" folder inside it. You should now have a folder for every artist in your iTunes library. There should also be a folder entitled "Compilations". Drag this folder into the "Library" icon in your iTunes window. Wait while iTunes copies all of your lossy songs back onto your internal drive.
6. If the "Compilations" folder you dragged in step 5 still exists, you
should be able to delete it, as all of its contents should now be in a similar "compilations" folder in the folder you specified in step 4. (Check this before deleting, though!!!)
You're done. All of your lossless files are on the external drive. All of your lossy files are on the internal drive and you shouldn't have lost your ratings, etc. (At least, these steps worked for me, using the latest version of iTunes.) You can even change the "part of a compilation" settings as much as you please.
Good luck!