Better late than never ... photos of the finished results attached.
The product used was "Westley's Brush On Rubberized Undercoating" shot with an Undercoating Spray Gun bought recently online from TCP Global Corp. From what I can tell, the Westley's product is no longer sold, but it appears to made from Tar (and god knows what else). Something like this, also from TCP might work, but don't just take my word for it, by all means test first:
http://www.amazon.com/Approved-Brushable-Asphalt-Bitumen-Base-Undercoating/dp/B002HOG28GI reduced the product with garden-variety Medium Speed Lacquer Thinner to about the consistency of a thick cake mix. If you reduce it too much, you'll lose the texture. If you don't reduce it enough, it'll only sputter out of the gun. The air pressure needs to be set HIGH in order to move the heavy material. I found 100 psi about right. Less causes more sputtering, more causes the overspray to really scatter. Best technique is to set the pressure to 100psi, add a little lacquer thinner to the product, stir in well, and spray on a scrap test panel. Repeat adding thinner, stirring, test spraying until satisfied. When first sprayed, it'll be very "peaky" looking, but know that as the solvent dries out of it, the peaks will flatten out to quite a degree. So, you want to shoot for as much texture as possible.
No matter what you do, this stuff spatters and cleanup is a drag. Protect yourself and your working areas. You are warned.
Once the stuff is applied to the car, leave it alone for a day, then follow up by cleaning off excessive spatter. Factory original sound deadener didn't spatter/speckle like this stuff does, but the overspray can be cleaned off with Turtle Wax Bug & Tar remover to bring things back looking right. The remover worked amazingly well and doesn't harm paint. Just don't leave the sound deadener to dry more than a day before cleaning up the overspray or you'll have a real difficult time, but you also don't want to get in there cleaning much sooner unless you want to ruin your work and end up with it coating you too!
It took about 1/2 gallon to do both sides, but I applied it pretty conservatively as that's all I had to work with.