As CFI noted, this issue has nothing to do with duty. Your duty period is assigned by the company, or is that time when you are performing duties assigned by the company. Outside flying has no impact on your Part 121 or 135 duty times. Flight instruction has been defined by the FAA Chief Legal Counsel as "other commercial flying," and does count toward your flight time limitations.
This was recently discussed here, and several legal references were provided. Do a search.
Additionally, the FAA will pursue enforcement action under 91.13, Careless and Reckless Operation, where an inspector may deem your flying to present a safety hazard.
Regardless of your operation, be it strictly Part 91, 121, 125, or 135, you are still beholden to Part 91. You always operate under Part 91...the other parts merely add to it (and occasionally modify it, but generally just add restrictions). You area always subject to 91.13, and in each legal interpretation provided by the FAA Chief Legal Counsel, the issue of 91.13 has been raised. Yes, you can do your 14 or 16 hours and then go instruct...but be sure that you don't raise a safety issue by doing so. Remember that w(h)eather your flying poses a safety risk won't be decided by you in enforcement action and the appeal process...so act accordingly.
The central question in this thread asks if flight instruction for no compensation is commercial flying. According to the FAA, as specifically noted by the FAA Chief Legal Counsel (representing the FAA Administrator), the answer is yes...flight instruction is "other commercial flying" for the purposes of flight time limitations under Parts 121 or 135.
One thread in which the matter was recently discussed:
http://forums.flightinfo.com/showthread.php?t=70354
In that thread, the Chief Legal Counsel notes that "other commercial flying" is any non-military flying for which one receives compensation. This returns the original poster to the question of w(h)eather the uncompensated instruction counts toward flight time limitations. It does not count toward duty limitations (unless performed for the certificate holder), but it can count toward flight limitations.
Is it indeed commercial in nature? Is the pilot receiving compensation? Is the pilot logging the time? The Administrator has held that the logging of flight time is compensation (regardless of w(h)eather that time is used toward any certificate or rating). If you log the time, it may be deemed compensation, and therefore commercial flying. Did you obtain any favor with the student? Did the student buy you lunch? Did you further your career by making a contact? Did you benifit in any way? The FAA may make a case that you are engaging in commercial flying. While this may or may not ever happen, the question is one of legality, and most central to this is what it means to you.
If you make yourself available for enforcement action, you migth be able to win on appeal before an ALJ farther down the line. However, your career may already be in shambles. Treat your flight instruction as commercial flying, report it to your company, and include it in your total flight limitations. Don't give anybody room to question you. The saying is avoid the very appearance of evil. If it appears like commercial flying, regardless of what loophole you might try to use to weasil out of it, it may be deemed commercial flying. W(h)eather or not you're exhonerated in the appeal process is really quite irrelevant, as the damage is already done.
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