Yeah, the after part to control order of operations is already in place.
To answer the other question, my language my have been imprecise or weird. It’s moot now. I can give you details when I email you some other notes, but TLDR: hacky crap that predates my involvement with this.
My remaining related question, though, is this: dependency 2, once built, provides a C library that ROS needs to build. The ROS side looks like this.
FIND_LIBRARY(SIMPLECAT_LIBRARY simplecat /home/myuser/simplecat/lib/)
What is the Snapcraft best practice? This makes it look like I can maybe copy them into $SNAP/usr/lib/ manually and then modify our ROS install to find them there, but all instructions I can find apply to runtime, not buildtime.