A natural extension to the existing radius API would be to allow setting specific corner radiuses.
For example, it could be named lv_style_set_radius_side(lv_style_t *, lv_state_t, lv_radius_corner_t, lv_style_int_t) and work just like lv_style_set_border_side(...) and friends.
One difference, however, should be that you can call lv_style_set_radius_side() multiple times using different corners without overriding other corners; e.g.,
lv_style_t half_round;
lv_style_init(&half_round);
lv_style_set_radius_side(&half_round, LV_STATE_DEFAULT, LV_RADIUS_TOP_LEFT | LV_RADIUS_BOTTOM_LEFT, 0);
lv_style_set_radius_side(&half_round, LV_STATE_DEFAULT, LV_RADIUS_TOP_RIGHT, 10);
lv_style_set_radius_side(&half_round, LV_STATE_DEFAULT, LV_RADIUS_BOTTOM_RIGHT, 5);