Hi OFWW
I Never used shellac for that but it sounds good and reasonable as long as 2 mating surfaces are flat. I use a small dab ot white Elmers , the cheap stuff.
I also make a lot of replacement nuts and saddles from Cow Bone.
Belt sand Some Bome scraps
Save the bone dust in a small tin below your belt sander.
Its handy for slot repairs when they are too deep.
Mix it with epoxy and fill the slot. Let cure then re-cut the slot
to a higher measurement . to avoid a buzz at fret #1
rick B.