I rather think that any low carb doughnut would be rather sad.  To borrow a phrase from the late great DNA you would end up with a substance that is almost, but not quite, entirely unlike a doughnut.
To be honest the best substitute for a doughnut is probably a doughnut.  OK, diabetics are never going to be able to eat many, but if your BG control is generally tight you should be able to build an occasional one in as a treat (especially if you eat it shortly before you know that you will be exercising).  It is probably better to do that than make them "forbidden fruit" with an absolute ban.