Brymon did have a go at a Dash 7 proving flight into ISC when they were looking to phase out their final Twin Otter (which ironically is still in service with Skybus today). They did it once, and it scared the Bejesus out of everyone and the idea was quickly dropped.
Just out of interest, why can't the 228 operate ISC? There is no element of airfield licensing or fire cover regulatory problem to prevent it, and the performance looks as though it should be adequate on paper, even taking into account the fact that a "balanced" field length doesn't really apply at St Mary's.