One of my acquaintances is half-deaf (i.e. she doesn't hear on one side). She remarked today that she just realized that she should have the music on her iPod in mono, which makes sense.
Your request makes perfect sense. If the user cannot hear with one of the ears and the track happens to have a significant stereo effect part of the content is lost. In addition, if the player is only in personal use it would be advantageus to save some storage space by encoding the content in the mono mode.
MC do various things on-the-fly when transferring to portable players (and I love it for it!). Could convert to mono be one of those? I wouldn't need it, but if it's not too much work, it'd seem like a cool feature.
Actually, you can easily do that by specifying mono encoding in the LAME encoder's settings. The command line switch is
-m m (which means "mode: mono")
For instance, if you prefer to use "High Quality Portable" you can first select that setting and then change to "custom". The displayed command line is what HQ Portable uses, i.e.
-V 5 --vbr-new. You can change it to
-m m -V 5 --vbr-new and it will encode in the mono mode.
The resulting files will not be half the size of the stereo files because also the LAME encoder's default stereo mode can effectively "combine" information that happens to be identical in both channels, but they will still be significantly smaller.
If the files are already in the MP3 or other supported format you must use the "always convert" mode in order to always force re-encoding.
EDIT
It might also be good to use a setting like VBR "High" (-V 2 --vbr-new) when re-encoding MP3 or other lossy file formats. "Lossy to lossy" causes always some additional audio quality loss which may become audible at lower quality settings/bitrates. "High" with the mono switch would probably result files that are approximately the same size as stereo "HQ Portable" files.