I've been working for the airlines for several years now, and always had good intentions to keep up instructing, which ever materialzed. To be honest I decided not to, as especially after 9/11 I thought the last thing FIs at a local club would want is an off-duty airline pilot to come along, take some of their work, when they were looking at a long future in a C152. I know I would have felt frustrated in their position.
In the airline industry you are restricted to 900hours per year, does instructing eat into this 900 hours as it is for reimbursment or does it not restrict your airline hours? Are the airlines happy for you to have this side job?