I've got a similar sounding setup, with a rack and a rack trunk, and full fenders. Seatpost mounting is out, of course. My rack trunk has a blinkie loop, but the pocket it's on sags and the light ends up pointed at the ground. I've got a simple piece of bent metal bar attached to the rack platform and extending out far enough to mount my Superflash. It was all of 5 minutes work and $1.00 of materials at the hardware store.
For fender mounting, check out the Spanninga SPXb tail light. It mounts right to the fender, and runs about 100 hours on 2 AA batteries.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.