I'm not saying you are right or wrong, you are the one with experience knocking on doors. I'm just stating my views on the approach you are recommending and how my mind works when a stranger knocks.
To me, a stranger standing on my porch knocking is probably there to sell me something or to save my soul. Either way, I am not the least bit interested, I'll go to the store if I need to buy something and to the church if my soul needs saving, thank you. I'm certainly not going to be in the mood for some attempt at humor. I would rather this intruder get to the point quickly, he is interrupting whatever I was doing when he decided to knock and my attention span will be very short, no time for wise cracks.
And, I think it is ridiculous to assume that most people who put up No Soliciting signs are just "week" and have little sales resistance. They are not wanting the signs to give the no's so they don't have to. They want solicitors to stay away, they have no desire to waste their time with them. In my opinion, your approach shows arrogance and a total lack of respect for the property owner.
And, "awe, put your checkbook away, I'm not here to sell you anything"? In the first place, if a stranger is standing at my door, he is out of line, talking down to me, by telling me to do anything, period. He is not my superior and certainly not an invited guest. The visit would end right there for me.
I'm aware that not everyone has the same reaction to door knockers that I do. I just thought you might want to know how the approach you were recommending is sometimes perceived by the "victim" on the other side of that door.
No offense intended.