Hi ,
this would be a nice-to-have feature for some sensitive people, but not
important for 99% of the rest, I guess. 
Dont know how possible is to have this working, and dont know either if its
slimserver or squeezebox task to do it.
The idea is:
Absolut polarity (not AC polarity) is what you change, for example, when you
swap  the cables on your speakers leads.  
There are records that comes with one polarity and others with the reversed one.
Again most people dont sense it and dont care about it , but there are other
people very sensitive to it, and depending on the type of equipment it could be
easy or completely impossible to change the polarity for each record while you
are listening to it, without swaping cable leads on the speaker.
So, what I was thinking is something about a TAG on the music file stating "0"
polarity,  or "180" polarity, and the Slimserver or Squeezebox interpreting the
TAG, and "switching leads" in the digital domain. This (and FLAC decoding on
Squeezebox, and no gaps when listening to whole-album FLACS :-) ) could be
attractive to many audiophiles.
Again, I have ABSOLUTELY NO IDEA how simple/complicated is to change absolute
polarity on the digital domain... so this is just an idea for you to evaluate
for a possible enhancement in the future, if it does make any sense :-)

Thanks
N�stor