Impedance peaks? What are you getting at?

Since when is a speaker a perfect resistance at all frequencies? And your parallel-resonant circuit is reactive as well, it will provide a near resistive load at the resonant frequency, sure, but it will still be capacitive reactive at lower frequencies and inductive reactive at higher frequencies - so what is the concern here?

Blocking caps have been used in the output of single-ended audio amplifiers for decades, since the invention of the transistor, in fact.
What the original poster proposes is nothing new, nor does it violate any "rules" - he simply did some homework for us and published the results.

I am sure you mean well, but as has been pointed out, this mod is over four years old and I have yet to see a thread where someone has destroyed their amp by putting a capacitor in a speaker lead.