Critically, if someone within the org volunteers to organize and manage the cross departmental knowledge they'll be shut down by people within the company for trying to accumulate power or stepping into someone else's turf.
Only an outsider can safely talk to everyone. They aren't a threat because they aren't part of the company and therefore can't take over.
It cannot be volunteer work. It has to be sponsored from the top. If it is, then a team of qualified FTEs can do this job similar to an external consultancy. But rarely will old-timer well-settled FTEs will leave their current position and take on this temporary risky gig.
Also, the end result and the path to get there isn't clear for FTEs – let's say they did the functional domain requirement gathering work – then who will implement it – they will have to contract out the development work – they may not be empowered to think and see through to the end goal fully.
On the other hand, for the top management folks – it is easier to temporarily empower the external consultants and terminate the engagement with them at any time – but they cannot do the same with FTEs due to employment laws etc.
What if it wasn't temporary, but just part of the CEOs office? The first goal should be to actively prevent these foul-ups by keeping open lines of communication.
I've always thought that a team like this is critical to keeping a company running. Any org that's 1000+ needs a 5-7 person team outside of any department that the CEO can throw at problems, but no org that I'm aware of has adopted this structure.
Only an outsider can safely talk to everyone. They aren't a threat because they aren't part of the company and therefore can't take over.