tldr; use VLC + e.
Can not be done using omxplayer
I am kind of stating the obvious, but it looks like it cannot be done with omxplayer. A quick search results in the following the keyboard shortcuts that are related but do not offer a solution.
Left Arrow Seek -30
Right Arrow Seek +30
Down Arrow Seek -600
Up Arrow Seek +600
Also, starting up the process at a specific frame is not possible, because it is limited to precision in seconds.
Usage: omxplayer [OPTIONS] [FILE]
# omitted other options for brevity
-l / --pos start position (in seconds)
Can be done using VLC!
I just stumbled upon the topic VLC Hardware acceleration on this RP Exchange. This enables you to run VLC on your Raspberry Pi. Since VLC features frame-by-frame navigation (since 2008) you are saved, just press e!
Command line options
Just like omxplayer, there are also keyboard shortcuts available - and configurable at command line startup - for VLC but again only with seconds as time unit.
--extrashort-jump-size=<integer [-2147483648 .. 2147483647]>
Very short jump length, in seconds.
--short-jump-size=<integer [-2147483648 .. 2147483647]>
Short jump length, in seconds.
--medium-jump-size=<integer [-2147483648 .. 2147483647]>
Medium jump length, in seconds.
--long-jump-size=<integer [-2147483648 .. 2147483647]>
Long jump length, in seconds.
Since this doesn't really work, you could also start VLC
x is the amount of milliseconds. This starts the video on the defined point in time.