I don't think you're understanding the question folks. It sounds to me like the OP is not using a rack but would like to utilise the rack braze-on at the top of the seatstay to mount a light.
I also looked into this recently but drew a blank. A shame really because if there's no rack it would be a brilliant, logical position for a light. Same for the front fork carrier braze-ons.
Ah ha, gotcha. Piece of cake. You just need a slightly longer M5 allen bolt, a regular small washer, one of the crimped type locking washers and a suitable 2 piece light bracket that will accept your light but discard the seatpost piece.
The remaining light mount piece will judt bolt into the M5 hole with the locking washer in between, this holds it steady against the knurled edge of the light mount so it doesn't sag on bad roads. The regular washer needs to fit inside the bolt recess of the light mount so when you run the M5 bolt through and tighten it, it doesn't eat into the plastic.