Good tailored men’s jackets are usually closed off at the shoulder. The sleeve is hand stitched in which makes it impossible to get to the body of the jacket through the sleeve. In this case, open the side seam and do the work you have to do, then when you are ready to close, open the bottom of the breast pocket. Pop the side seam through and sew up, so that noone will know you opened this seam. Then stitch the bottom lining together. The stitching will be on the opposite side, but really no one will know that you did this, and usually people don’t put big things in the beast pocket anyway.
Judith aka genie