"I have a slow leak (a few drops per hour) in a tub drain seal."

The drain to the tub itself or one of the joints in the drain line?

If the seal to the tub itself is leaking, try tightening the nut.

If the leak is on the drain line, a seal is often upside down.

The leak is at the connection between the drain shoe and the bottom of the tub. A black round gasket is compressed between these two when the drain basket is tightened (yes, with a drain wrench).

I've tried tightening the drain basket, re-doing the plumbers putty around the basket, and putting 3 wraps of red (pink) teflon tape on the basket threads, and, yet, it still leaks very slowly. Would coating the black gasket with silicone sealant work?

TIA

"Would coating the black gasket with silicone sealant work? "

Only until the silicone looses its bond to the tub's exterior from repeated thermal expansion and contraction of the tub.

Gaskets are use in place of plumbers putty, rarely with plumbers putty.

A thicker and softer gasket would likely be a better choice.

i know its been a while but what did you do to fix this? I have the exact same problem