Playrate envelope with negative playrate abilities are heavily used in v
ideo softwares for time remapping of videos.
That's why it could be interested to have that kind of functionnalities in an audio software
Meanwhile, if you want negative...
Scratch it 
And for positive only, you already post a better solution :P
(if there is a already feature request for playrate/pitch envelope positive and negative... I'M IN !)