I just had another thought; the hardware I have right now is a two-piece thing; clip to hold light, clamp to grab seatstay, held together by a bolt/nut that is oriented in the left-right direction. I could just cut a small rectangle of scrap wood, drill two holes and bolt it to the rack-mount, and then drill a sideways hole between the mounting bolts to bolt on the clip!

Option 5, cost is 0 (assuming I have appropriate-sized bolts/nuts/washers around), that is what I will do. I might even have some black plasti-dip so the wood won't look quite so ghetto (and won't rot if it gets wet). I'll try to remember to come back with some pics when it's done...