Kam and I stopped in the bookstore last night after dinner, and I saw a book titled I Should Be Extremely Happy in Your Company: A Novel of Lewis and Clark, by Brian Hall.
Sadly, on closer inspection it turned out not to be a Lewis/Clark slash novel. (At least, not as far as I could tell from the info provided on the front and rear flaps.)
Furthermore, fanfiction.net doesn't appear to provide any Lewis/Clark fics. (They do, however, have a large collection of C.S. Lewis fanfic.)
Clearly this is a niche waiting to be filled.