"ATP" fired this volley in
:

There is an economy type
plastic shower drain pan which appears to be fitted with an integral
drain. 2" galvanized pipe is leaded or epoxied into the drain and
there is a substantial leak where this short piece is threaded into an
elbow.

Never seen one with an "integral drain", unless you just mean a declivity
in the plastic where a drain flange and coupler may be fit from BOTH
sides, and screwed tight.

I seriously doubt a close nipple was leaded to a plastic pan...

I think I would investigate from the top, perhaps nibbling away the
strainer, if it won't remove easily. There must be a 'bulkhead' style
coupling there (two halves, clamping on opposite sides of the work, with
the top-most piece screwing into the bottom tailpiece).

I cannot imagine the plastic (which is usually vacuum-molded) being also
extruded into a proper tailpiece. Maybe a tailpiece was glued on from
the bottom?

Regardless, if there's enough meat around the opening, there should be
some sort of drain coupling you can buy to fit, even if you have to cut
out the old one -- keeping as small a diameter hole as you can muster.

Lloyd