nice!
The offset file will be a json file so you could potentially read it like this:

{:ok, body} = File.read(log.offset)

Then you should be able to decode the body with the Jason library.

While testing this for myself I noticed that my computer had a lot of issues reading the huge file however.
Alternatively you could execute a command on your file-system instead (through a .sh script) with grep and sed to grab strings from your offset. You can see an example of sed here:

https://git.picodevelopment.nl/hendrikpeter/tmux-mac/blob/master/segments/weather.sh#L61 (I'm reading "icon": "someicon" in that line there)