I would not get hung up on making a cookie for a group of elderly diabetics.
Concentrate on making a great cookie that everyone can enjoy.
I would look for recipes that get flavor from citrus zest, spices, extracts, etc... and just drop the salt.
I would also look for high fiber ingredients as Kathleen suggests. Try fiddling around with a recipe like this one.
Cranberry Oatmeal Cookie Recipe - The Artful Parent
Cut the sugar in half, drop the salt, cut the cranberries by half and chop them to release flavor, add some finely chopped walnuts, leave off the sprinkle of sugar etc...
Pick up the Valentine theme with the splash of red from the cranberries, the heart shape and some inexpensive red, heart themed, packaging from the dollar store. Sell the sizzle, not the steak!
You have plenty of time to experiment and it doesn't sound like you will have trouble assembling a tasting panel!
Good luck and have fun!