Would it be possible to divide a move into 3 steps (when using retraction). When moving to next position start the move and retract at the same time, than move most of the way as usual and than start feeding the filament back during the last part of the move.
Eg. if moving from 0,0 to 0,100 move from 0,0 to 0,3 while retracting, move from 0,3 to 0,97 as usually and finnaly move from 0,97 to 0,100 while reversing filament. For short moves just move 1/2 of the distance while retracting and 1/2 of the distance priming again.

The combination of retraction and travel move is only possible with firmwares that correctly implement the G0 command. If you supply the --g0 option to Slic3r it will emit G0 instead of G1 for travel moves and retraction, and will combine them. Unfortunately, neither Sprinter nor Marlin implement G0 correctly so you'll get a very slow and ineffective retraction because it will be syncronized with the travel.

On the other hand, I think that starting to unretract before finishing the move would defeat the purpose of retracting because you'd push some plastic out of the nozzle before being at the target destination.

Sound Wrote:
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. Unfortunately, neither Sprinter nor Marlin
> implement G0 correctly so you'll get a very slow
> and ineffective retraction because it will be
> syncronized with the travel.
>
That wouldnt be a problem if slic3r make the movement into 3 separate moves, where the lenght of first/last part is based uppon the amount to retract/unretract.