For forms of the form $Ax^d - By^d$ this seems to have been done by Bennett, Dummigan, and Wooley:
Bennett, M. A.(1-MI); Dummigan, N. P.(1-MI); Wooley, T. D.(1-MI)
The representation of integers by binary additive forms. (English summary)
Compositio Math. 111 (1998), no. 1, 15–33.
11D85 (11D41 11P05)
M. A. Bennett, N. P. Dummigan, and T. D. Wooley, The representation of integers by binary additive forms, Compositio Math. 111 (1998), no. 1, 15--33.