If YOU own the townhouse, then, the HOA cannot in most cases limit WHERE you can mount your dish. There are exceptions, provided in the FCC ruling, but, those are few.
The only restrictions that can reasonably be placed on customers regarding dish placement if from those who rent.
However, the chimney is a very poor place to mount a dish over 18" especially an HD dish. The wind loading is way more than you would think. A chimney is not built to withstand the substantial wind loading that would be placed on it by an HD dish. DirecTv understands the mechanics of this, and likely has placed this restriction on mounting in order to avoid being sued for property damage.
All that being said, the HOA, in most cases cannot limit where you mount a dish up to 39" in diameter.
If I were you, I would let the installer determine the best location for LOS and go from there. I think there are avenues of persuit for consumers who are being oppressed by HOA's with regard to dishes or other antennas.
Like has already been said, they cannot limit you to the chimney for mounting, and for that big a dish, you would not want to use the chimney anyway.