By definition, repression requires one to push down "unwanted" extroverted thoughts. This would normally imply negative conditioning, causing someone to push down their outgoing nature. I believe that some could even call this introversion, actually (the degree of E and I, in theory, could be based upon the degree of "reward" factors to speaking out relative to the degree of "punishment" factors to speaking out).
In any case, as with any represion, you'd be looking for outbursts as stray thoughts break out... and repression merely transfers the nature of things - if E is truly biological, as compared to I, and the behaviour it would manifest is being repressed, there will be an outlet, somewhere. It's impossible to know what it would be, for it would be different with every individual.