One specific thing about MAN that might make it difficult is that it has flights to the US from all three terminals - that would surely require 3 separate facilities no?
I'm sure the US authorities will only want one facility due to cost reasons; that's why the DUB one doesn't cover flights later in the day. That means that it will be impossible to fit all the US flights in unless they are moved to the terminal with the facility and what does that do to terminal capacity?
Then this US sterile area will be empty for 16 hours a day.
If I remember correctly, the DUB facility is flexible, so that when the US bit closes, most of the area can be used for "ordinary" flights
Apparently the UK govt has given the green light for this, as the list is only of countries that the government has allowed armed US customs officers on their territory, and, the airports are ones that have bidded for the USPBC and the DHS has deemed the airports suitable to house the facility.
It certainly used to be the case that there was a need for the UK to enact legislation to allow USPBC to take place on UK territory. Not sure whether this is still the case, but it would appear if LAX-LHR is correct that at least progress is being made now.
And it is encouraging to see that again according to the above quote, that MAN have made a bid and US DHS has approved it. You see Bagso, you never know what is going on in the background out of the public domain...
So if MAN has made a bid and it has been accepted, they obviously have a cunning plan in place to do this, so we need to await the details. Whether we will have to wait for the terminal redevelopments or some thing will be possible in the shorter term remains to be revealed. But as other posters have pointed out, this gives an ideal opportunity to provide a proper integrated facility as part of whatever the re-development is.