Every time building a OpenMCT image and flashing it up to MCT is boring when you just want to check if your code/binary works fine. The solution is to put a writeable layer above the r/o path structure using the bind feature.

First copy the content of the readonly path structure in a writeable position. Assuming you have your hdu mounted to /usr and you want to edit inside /sbin. First create a 'working directory'

~ $ mkdir -p /usr/work

Then copy the content of /sbin to the 'working directory'

~ $ cp -a /sbin /usr/work

Then mount /usr/work/sbin above /sbin

~ $ mount /usr/work/sbin /sbin -o bind

Voila... edit and have fun. If something goes wrong you have your modifications still in /usr/work/sbin and a useable /sbin to boot with.