Ich bin auch auf der Suche nach einem solchen Tweak aus Cydia etc.! Eigendlich müsste es gehen da es im Appstore auch ein solches App gibt aber halt nicht in die normale Kamera App eingebunden.

https://itunes.apple.com/de/app/slopro-1000fps-zeitlupe-video/id507232505?mt=8

Und die Optionen folgender Maßen einstellen: •Tap on EDIT from top-right •Select the Clip In and Clip Out parts to select the portion of the video that you want the slo-mo applied to. (Position the seeker where you want to start editing and tap on Clip In. Repeat procedure for Clip Out.) •Tap on the Slow-motion options on top. You have a lot of options to play with, but the 500fps and the 1000fps are the key players in creating ultra-slow-motion videos. •Once you've watched the preview of the slow-motion, it's time to render. •Tap on Render and select Optical Flow for the best results.