This feature already exists: with the SD recording in event-only mode, tap on the < or > icons on either end of the playback timeline. This will jump to the previous or next motion event.
Also, you can use a two finger pinch-out gesture on the timeline to zoom in for finer control when scrolling manually.
There’s a wishlist topic for this here: Mark Events In SD Card Playback
Another related wishlist topic: Jump to user-entered time (hh:mm) in SD playback timeline