Hi,
It all depends on where you want to introduce the variation.... If you by "detune" the oscillator mean to tweak the OSCTUNE register slightly between devices then you might as well program in a different seed from the get-go - there will be a unique program in each device either way.

Perhaps you can use something like the RCTime command with an external RC (duh...) network to generate the seed and rely on the tolerance of the R and the C to get the needed variance.

/Henrik.

EDIT: Art beat me to it while I was typing and doing some other things.