My simple game plan would be to:
1) Use a mill to drill the holes in a piece of aluminum (actually probably into the edge of a 1/8" thick piece of aluminum)
2) Create an easy to disassemble "mold from more 1/8" aluminum
3) Fabricate pins with a lathe
4) Solder resistor/wire/pins (and put the heat shrink on in prep for the last step)
5) Put said parts into the mold
6) fill with epoxy.
But there is a key improvement to make to this. The issue I've had for a while is that the epoxy-wire interface fatigues over time and necessitates replacement. Getting the DAI pins out of epoxy is not very easy.
1) Create mold similar to above
2) wire the pins to a resistor and 2-pin connector (I have these; some tiny JST connector but I don't recall the part number)
3) Fill with epoxy
This would be a permanent (as in never have to remake it) connector to the hearing aids.
4) Hook up the male end of the JST connector to the wire going to the 3.5 mm jack. (solder, epoxy, etc; doesn't really need a mold)
5) Connect the two parts, hot glue them together, and shrink wrap.
So then when fatigue issues occur, remove the hot glue, and replace the wire portion. The wire and JST connectors are easy to buy!
The big question is what material to use for the pins. Brass is too soft (bends), unsurprisingly. Steel would need to be plated (ehh-effort).
Todo: investigate alternate copper alloys: http://en.wikipedia.org/wiki/Copper_alloy