My biggest question is WHERE is the sewing room? On an UPPER level? If so, that may be the problem. I don't know of anyone who has an upper level that isn't normally hotter than the main level or basement area. Sometimes bad in the winter too (they have to close the registers to keep the furnace heat out of the upstairs).
If your sewing area is upstairs, maybe you can close your central air vents in the lower levels and send it alllll upstairs.
Good luck