so, apparently, your bones have androgen receptors and estrogen receptors. in a genetic male, bone resorption can happen via either/or - and your body will actually convert testosterone to estrogen using an enzyme called aromatase, in order to bind to the estrogen receptors.
a genetic male with decreased testosterone would consequently have less hormones to bind to the receptors, but that binding can happen via either hormone. so, the conversion doesn't seem to be total, after all. i would have to suppose that there is minimal, if any, sexual dimorphism in bone resorption, and women build and maintain bone density using the same process - just with more estrogen reception and less tesosterone reception.
logically, then, it shouldn't matter if you give your bones androgens or estrogens, so long as you're not decreasing your total hormone levels.
and, it's interesting to note that you can convert testosterone to estrogen, but you can't convert the estrogen back to testosterone, again.