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Wouldn't be trying to skid, just want the general forward-going benefits of riding fixed but with a variable ratio.
MechaMorgan-
you inevitably apply reverse torque on a fixed drivetrain, before cornering etc.
fixed hub gear as sig suggested is the way to go. -
The 'forward going benefits' wouldn't apply I think. On a normal fixed, when the cranks are at the deadpoint at 12 and 6 o'clock it's the 'lower' part of the chain which pulls the cranks through the deadspot. You wouldn't have this 'pull-effect' with a derailleur. The sturmey archer would be the easiest solution: http://www.sturmey-archer.com/en/products/detail/s3x-silver Alternatively, you could develop a new system on the basis of the old campagnolo paris-roubaix system which 'works' without a 'derailleur' (technically, it still has a derailleur but not in the sense of a modern derailleur which adjusts the chain slack):
Rodolfo
Sig_Arlecchino
I guess the rear mech would slam into the casette when you 'skid' or exert force against the pedal rotation. Afaik, the only way to combine gears and fixed is with hub gears. Sturmey archer offers a 3 speed fixed hub.