Everything seems to be working now.
The script overwrote my existing post observation script but after editing /usr/local/bin/satnogs-post to call my script as well as the gr-satellites script, everything seems to be in order. gr-satellites logs the following:
gr-satellites: Observation: 4269435, Norad: 43743, Name: REAKTOR_HELLO_WORLD, Script: satnogs_reaktor_hello_world_fsk9600_decoder.py
gr-satellites: Starting observation 4269435
gr-satellites: running at 57600 sps