MP3s also combines lower frequency left and right channels on the assumption that the lower tones don't contain directional info

I'm going to pick a small nit here -- this is misleading. Lower channel frequencies can be combined in Layer III for better compression, but there's no loss of stereo imaging here; essentially the channels are encoded as L+R and L-R. This is called middle/side or M/S joint stereo. Both channels can be reconstructed perfectly.

Layer III can also optionally use something called intensity joint stereo, in which some high frequencies are encoded as mono plus some stereo imaging information. In this case you can't always reconstruct both channels perfectly. This might be what you're thinking of, as it's the only joint stereo option for Layer I and Layer II. However, it's not very common in Layer III.

For Layer III, either of these joint stereo options, both, or neither are possible. M/S is probably most common, as there's usually little reason not to use it. I have found inconsistencies, however, in the way some decoders handle a combination of M/S and intensity joint stereo, so I don't recommend anyone use both.

-v