attaching seatpost acessories to rear of racks? most night lights are designed to attach to the seatpost. but while carrying a bag on the rear rack that area get's blocked.

some manufacturers sell cheap, awful, lights that attach to the rack (topeak) but i figure some kind of adapter would be much more useful (mostly since everyone may have night lights before getting a rack... i do :)

but my searches for such a adapter failed. do you know any?

This is one example of night light i have
http://www.dealextreme.com/p/rechargeable-7-mode-red-5-led-safety-bike-tail-light-with-7-mode-2-red-laser-beam-35mw-90117
(i regret. do not buy, laser is useless snake oil. but the leds are OK. EDIT: regret #2, i had rain for the 1st time --socal-- and when wet, the shorted the buttons which are obviously not water proof, so it spend all night turning the leds and laser randomly on and off. battery hold up ok though.)

and this a rack
http://www.blackburndesign.com/racks/mtn-2-rack.html
(it has a hole on the back, where you can fit a L shaped adapter that holds a reflexive cat's eye)
One possibility is to use extra rubber shims to use the seatpost attaching mechanism to attach to the stay of your rack, like so:



You might be able to use something like the Low down light mount bolted through that hole to give you a little bit of post hanging from the hole and use that with the seatpost mount...

You might be able to mount the hardware without the seatpost mechanism to the hole of an L-bracket mounted in that hole...

With racks with a more serious light mounting bracket instead of just a horizontal hole, I've found many light can clip directly to the bracket with no extra hardware at all, like so: