My situation is a little different than yours but very similar at the same time.

I actually work for two 135 companies. When I got hired at my new company my old one allowed me to stay on part time as a training captain, which is done part 91. From what I have been told from my old CP, who coincidentally works at the same new company as me, is that all instructing counts towards flight time limitations and that the instructing is not duty time, however, it is not rest either. Meaning that you can pull 135 duty for the max 14 hours and then instruct for 6 hours(so long as you aren't flying more than 10 hours) but before you start 135 duty again you must have your 10 hours of rest.

I don't know if that's the FAA interpretation on the matter or not, but by adhering to that it will keep me out of trouble with the Feds.