Maybe you don't have to disassemble the head. I was thinking about having this done to mine and this is what I was going to do because I didn't want to disassemble the head.
I have a spare window from a pelican M6 that should be a direct replacement for the 27L head. I was going to swap windows and just have the head blasted with this window installed. I wasn't worried about destroying it(it is a spare). Now the window retaining o-ring will most likely get mangled during the blasting process, so order a new one from the shoppe.
This to me would be much easier than taking the head apart. If you want the window, shoot me a PM with your shipping info and I'll get it sent out tomorrow...