It is indeed illegal on the Stilly or any other fly only water(in
Washington anyway).
If the weight is incorporated into the fly it is legal. You will see guys
on the Stilly fishing heavy weighted fly's that are tied on jigs hooks and
floated under large strike indicators. While this is sometimes an
effective way to fish it's not as fun as hooking a fish on a sinktip(or a
surface pattern.....but few things in life are as good as that!).
When I first started fly fishing for steelhead I used lead core shooting
heads. These got down deep and fast but were a pain in the ass to cast. I
still always carry one shooting head with a loop on the end in case I need
to loop a small length of lead core to the end. I don't use it much,
especially on the Stilly.
I'm taking the day off tomorrow and fishing the Skagit for dogs or
anything else I can fool.