Players should note, bringing Mornington Crescent into bounds during an out-of-bounds game is strictly against the rules, and so is winning. Therefore, one must employ a level of mastery of the rules far beyond that required of many other variants, or simply tire one's opponent into conceding.
It is worth noting that Mornington Crescent is always considered out of bounds, as winning moves can only be played when proper conditions have been met. Formalizing the 'out-of-bounds' convention may be handy in prolonging games, but this rule-set is essentially indistinguishable from so many others.
I believe the distinction is that in a game of Mornington Crescent Out-of-Bounds, one must first explicitly play a move that brings it into bounds – and anounce the fact – before later playing a winning move. In most other variants, bringing Mornington Crescent in bounds may be done implicitly, thus allowing for the possibility of 'surprise' victories.