My understanding is that any instructing is carried out on the CPL and therefore places you under the 900hr/year ruling so I would check with your employer as to where they stand on you working under your CPL elsewhere. Even in the instructing world most FTO's forbid us to go off and instruct elsewhere on a temporary or weekend basis as most want their pound of flesh (900hrs) to themselves. My hunch is most airlines will feel the same unless you're with a good'un.
Also, it is my understanding that you are restricted to a maximum of 100 hours per month on a CPL, no doubt someone will correct me on that if I am wrong?
VFE.