A built in scrather would be nice…
Just an idea 
well it could be a “move to sample offset” command, where you would put in an offset and renoise would play thru the audio to get there. there could be different curves to set as well, to get different sounding “scratches”. i don’t expect to ever see this, unfortunately.
It looks like idea of improvement PITCH envelope in Instrument Editor window.
The present one used to change pitch when sample playing in forward direction.
But if extend axis to both forward and reverse directions - you can play picth as scratchin’ turntable! (something like this i saw in hip-hop ejay) 
the solution is to first learn how to scratch and then use renoise to move the sample around like a record (forwards backwards with 0bxx pitch-up pitch-down 01xx/02xx) while using 04xx instructions to operate your “crossfader”. if you do a search for “toast” on the forums within the last 2 months. i posted a link someplace to a renoise file that demonstrates this as part of a non-sequitur i got involved in with sagosen. i think it was in “off topic”. good luck.
ps: renoising scratches is fun. part of tracking is building stuff like this using the tracker’s interface. it’s totally possible. if renoise did it for you there really wouldn’t be much of a point to having a pattern editor 
Hope you don’t mind, but I dug up that RNS that you posted. It’s actually very well excecuted. I’ve never thought of trying to do that! I think it’s a bit easier just to sample some real scratches 
Here it is: Toast
Great example ! 