In this case I think it is as simple as knowing there is such a function in a different package:
require(latticeExtra)
?useOuterStrips
wr <- wireframe(z ~ x * y | cond1+cond2, data = g, groups = gr,
useOuterStrips(wr)
(It's not quite as you described but it's pretty close, and I don't think it makes sense to have "only one value" on the outer strips, when the usual case is to have different levels in the conditioning variables.)