Is Gordon Brown as secure as PM as we all thought?

For a while now the consensus opinion has been that a Labour challenger to Gordon Brown would need to garner the support of a fifth of Labour MPs. The FT have found a possible way around this:

"There is a Labour rule that would allow for a change of prime minister without the immediate need for a party leadership election. It is effectively the “silent coup” clause.

If the cabinet joined forces and convinced a serving Labour prime minister that his time was up, he would become “permanently unavailable”, in the words of the party rule book. The cabinet, “in consultation with the NEC”, would then be able to anoint a new prime minister “until a ballot…can be carried out”. An election is not immediately triggered: the cabinet could decide to “leave the post vacant until the next party conference”."

The party rule itself is:

" 4B.2e Procedure in a vacancy

(i) When the party is in government and the party leader is prime minister
and the party leader, for whatever reason, becomes permanently unavailable,
the cabinet shall, in consultation with the NEC, appoint one of its members to
serve as party leader until a ballot under these rules can be carried out.

(ii) When the party is in government and the deputy leader becomes party
leader under (i) of this rule, the Cabinet may, in consultation with the NEC,
appoint one of its members to serve as deputy leader until the next party
conference. The Cabinet may alternatively, in consultation with the NEC,
leave the post vacant until the next party conference."

So the Labour party do have a "men in grey suits" clause, but can you imagine them using it? Under what circumstances would Gordon Brown be "permanently unavailable" and could he ever be persuaded so to be? Would the Country stand for a second unelected Prime Minister being foisted on them, one whilst he and his party were popular was acceptable; I doubt it would be as easy with Labour in the doldrums as they are now.