Actually testing the effect of trim tab position on tail down force being exerted in flight would require that the elevator be firmly held in a fixed position. Doing this by holding onto the yoke is problematic and probably won't yield a valid test result since the pressure required to hold the elevator fixed would be changing with the actual trim tab position and would probably result in a small amount of elevator movement in spite of one's best efforts. I tried it once following a similar discussion and didn't believe I was actually holding the elevator in a truly fixed position. Nevertheless my initial impression was that even if I were able to hold the elevator fixed, the effect of changing the trim tab position would have only a small effect upon the resulting pitch attitude of the airplane. It's my impression that only a small amount of influence upon the pitch attitude of the airplane would be possible using trim tab alone with a jammed elevator. Perhaps together with shifting weight within the cabin, changing configuration and power, using the trim wheel might be one factor in providing enough pitch authority to keep the airplane under some modicum of control.
Looking at it from another perspective the surface area of the trim tab compared to the surface area of the elevator is likely in rough proportion to the amount of tail down force change which the trim tab is capable of producing compared to that of the elevator. The effectiveness of the trim tab in changing the neutral force position of the elevator relies upon the distance aft of the elevator hinge line and the amount of aerodynamic force generated by the trim tab deflection. (arm X weight = moment) The aerodynamic force generated by the diminutive trim tab surface area with the elevator being held in a fixed position would be tiny compared to that of the much greater surface area elevator capable of being deflected.
Still it would be interesting to go up and flight test this assertion and put the question to rest once and for all! It would be necessary I believe to construct a clamping device which could be installed on the yoke to positively lock the elevator in a fixed position so the experiment could be carried out with some level of assurance as to it's validity. Of course the locking device would have to be quickly and positively removable even under high load conditions. Who would go to the trouble and expense to find out?
So I'm just going to operate under the presumption that while the trim tab might contribute somewhat to controllability with a jammed elevator, the other measures mentioned above are also available in the event they are required.
Now on to considering a broken cable or other control system malfunction...
Happy flying,
westhawk