I have a Zircon stud finder that works fairly well on sheetrock walls but even though it has a deep setting it is totally useless on plaster walls. I also have a good magnetic stud finder but it is generally not useful. Is there any alternative to drilling lots of tiny holes and then patching lots of tiny holes?

The best way is to use a metal detector to find the nails holding the laths.

I have a small hand held one that detects metal (and voltage), personally I have found stud detectors largely useless!
Cheers

Work from an outlet. Determine which side the box is on. U can do this by taking outlet out of box and using an awl (coat hanger if u don't have one) to find stud through hole in side of box.MAKE SURE U TURN POWER OFF TO BOX BEFORE STARTING. Measure back from edge of box 1 inch. This will give u center of stud.

From this mark measure 16 inches. This should give u center of the next stud. Unless framing was done in off pattern, then framing could be 18 inches.

Another way would be to remove cap molding on top of baseboard and drill holes it this area. After finding studs put molding back on hiding the drilled holes.

Chuck