Decided to purchase the ADEM "2H" headrest for the Xstream Hoagie seat, since the XL frame essentially allows full recline to the last seatpin hole...
 
The headrest assembly fastens to the seat through three standard bolt holes (like water bottle cages) spaced across the top of the seatframe.  It was a real task getting to the hole at the top of the seatframe -- you essentially have to go through three layers of fabric there, because the two side panels overlap there, as well as the reinforcing piece.    
 
 
 
 
Didn't have a soldering iron, so I xacto knifed / scissored it, with the addition of a butane lighter...
 
Here are other attachment pics
 
The top bolt is "inside" the top sleeve, which Don provides as well, already placed.
 

 
With headrest inserted -- this is a very nice "tongued" velcro attachment design, keeping the headrest insert where you put it, yet easily adjustable.  There is also a side setscrew to help eliminate any rattle.
 

 
Side view (after bending the support)
 

 
With the Hoagie seatbag installed...  the hole in the top of the bag is marginally large enough to get the headrest piece through -- it's like putting a difficult tire on a rim...  but I got it...
 

 
 
I also wound up "custom bending" the headrest extension, clamping it in my workshop vise -- as it was, it came too far forward -- but this is a typical adjustment, and the included instructions talk about this.
 
I used it on three rides last weekend, and it works quite nicely...

Good looking setup.

Likewise...  Still fiddling with it a bit.  Definitely better than not having one, and I'm pleasantly surprised by the lack of vibration transfer.
 
Got any photos of your replacement pad, and details of how you switched it?