Something I love about BL is that you find just about any premise you want in it...and I do mean ANY premise.
WILD ROCK (Wairudo Rokku), by Kazusa Takashima. First published in 2002 and first published in North America in 2006.
For years, Yuuen's tribe have been feuding with the tribe on the other side of the lake over their shared hunting grounds. It doesn't help that Yuuen is too slight and scared to land any big game. Thus, his father decides to have Yuuen pose as a woman to serve as a honeytrap for the other tribe's champion, Emba. The trick seems to work, but the more time Yuuen spends with Emba the more he finds himself falling for him. Eventually Yuuen can't take the deception any more, and his efforts to make his feelings known changes both their tribes forever.