Quite frankly, I think you provide a good illustration of wrong ways to use Knoppix6.X.

First, with grub, I don't have to perform that renaming game for kernels and initrds, just place them in different subdirectories. We don't know how much those names may be hard-coded either, so the safest is to keep renaming at a minimum. So, your kind of multi-booting is easily implemented with grub, but it seems from your description that it is way harder with extlinux. Your choice.

Second, it should be enough just to rename the KNOPPIX directory, e.g. to KNOPPIX644, and then use the cheatcode knoppix_dir=KNOPPIX644

Third, I try to minimize kernel graphics parameters, just using, for example, xmodule=nouveau, and doing the resolution adjustments in X. This is because this stuff has not been stable and fully reliable in 6.X, variable from release to release, and at least in part it is due to X changes. I think you may easily shoot yourself in the foot heaping up graphics parameters like you do - the graphics stuff just isn't that reliable nowadays.

Fourth, Knoppix is extremely flexible, provided you use the right cheat codes. But what do you do?
Obviously "fromhd" isn't any better since it still wants an exact location.
Well, simply using fromhd=<your single ext3 partition>, copying the KNOPPIX directory as KNOPPIX644 there, telling Knoppix about that with knoppix_dir=KNOPPIX644, and dropping that renaming game in extlinux should almost certainly give you a boot. You might even get it from USB, dropping the fromhd=, but keeping knoppix_dir=KNOPPIX644 on the command line.

I think one may even place the KNOPPIX directory in a subdirectory, and prepend that name in knoppix_dir, but I haven't tried.