Basically, we've been able to solve the issue using the servicemix lock - servicemix doesn't start when using the lock, so the embedded broker doesn't start either.
One annoyance note though:
we had to use:
because "servicemix" has been replaced by "karaf" on system.properties
this wouldn't be so annoying if we didn't have to guess it by ourselves - we couldn't find anywhere where this was listed. But all is well when it's well.