I remember a role-playing game that used hk for /x/. My idea of their reasoning is that they associated x with science fiction and so rejected it entirely, and liked the look of kh, but thought that perhaps players who were used to seeing kh in Earth languages might be used to it, and that where kh is exotic, hk would be even more exotic and perhaps more harsh-looking. I think this language was for a warlike race. The designers also provided an alternate pronunciation, "hack", for players who didn't understand the recommended pronunciation.
Ive been writing religious material for the religion of planet Teppala, and I feel the same way about the letter X ... i personally prefer to write /x/ as x, but I can't shake the association with science fiction, and therefore, since I hate digraphs, I have taken to using h for both /h/ and /x/. (The kh in the name Khulls stands for /kh/, and I dont mind digraphs for sounds that can be readily perceived as sequences.)