The question pretty much says it. Beyond knowing people that are interested in the same things, is there a collaborative reversing dumping ground for documenting specifically disassembly of closed source firmware?
I am not aware of a currently active generic "firmware reversing" forum.
A few years ago there was a pretty ambitious attempt with lostscrews.com but unfortunately it languished due to lack of attention, got overwhelmed with spam and eventually the domain has expired. I think the guys behind the /dev/ttyS0 blog also tried opening a forum a couple months ago but it wasn't very active and apparently has been closed down.
I guess the problem is that the area is somewhat nebulous and trying to cover everything won't really work. However, there are numerous forums that specialize in a specific type of firmware, manufacturer, or even just one product, and some of them are pretty big on their own. Here's a few examples that come to mind:
- XDA Developers: everything about hacking Android and Windows Mobile-based phones and other devices.
- PC BIOS hacking: Wim's BIOS and My Digital Life.
- Samsung TVs: SamyGO TV.
- Digital cameras: CHDK, Magic Lantern, Nikon Hacker.
- Ebook readers: MobileRead
- Audio players: Rockbox
- Wireless routers: too many to list here.
- iPhone/iPod/iPad: iDroid Project (and many others)
- and so on.
What are you looking for exactly? Are you looking to trade notes about specific firmwares, a place to trade tips and tricks when reversing low-level code, or just a place to show off your accomplishments?
I think the Hex-Rays forum would be a good place to start for a tips and tricks forum. It's currently not what you want, but it has the right people browsing the forum already -- which is generally the hardest part of creating an online resource like this. I don't know how amenable Hex-Rays is to posting complete write-ups and disassemblies of commercial firmwares.
For a place to show off your accomplishments, I believe that this StackExchange is appropriate (or /r/ReverseEngineering)!