In BrE, where we do use the word rowlock, those who are familiar with rowing and oarsmanship call it a ˈrɒlək (or possibly ˈrʌlək). Those who are not may instead use a spelling pronunciation ˈrəʊlɒk (compare forehead, where the spelling pronunciation has largely displaced the traditional ˈfɒrɪd).
LDOCE ascribes the spelling pronunciation only to AmE:
row•lock /ˈrɒlək $ ˈroʊ-/ n [C] BrE one of the U-shaped pieces of metal that holds the OARS of a rowing boat
In LPD I proceeded on the assumption that any American rowers who actually needed to use the British term would use a BrE-style pronunciation.
In a trivial way, the same sort of question arises if we ask what the BrE pronunciation of rowboat is. In a sense there isn’t one, because in BrE it is called a rowing boat.
But we have to draw a line somewhere: no one wants to say that in AmE trousers is pronounced pænts.