We pruned some old content from the Untamed Adventure website and, unfortunately, one of the casualties was my info about the seat post carrier I use and a buddy asked about it so let me post this here, on a more durable server location, for the sake of posterity . . . .
These pics from my Flickr site show the bike rack in action. I use a Swiss made carrier but you can buy them in the US (http://www.thorusa.com/pletscher.htm), I think I have the "System EasyFix" Master
from Pletscher, or one of their other heavier duty models -- it works well for my particular 29r bike and I can
put a really heavy pack on there. They have lighter ones you could
experiment with, too.
I know Sherry went to her local bike shop, with her loaded backpack,
and worked with them on a specific rack and combination that would suit
her bike frame and size. If you've got a good shop nearby it'd be
worth a visit.
In a longer race, shifting the weight off my back and onto the bike rack is a nice change and -- if necessary -- you can put a teammate's pack on their or something else if you end up needing to bike and haul additional gear. It's nice to have options when the race is long and the gear is copious!