Nice proposal!
I just find the use of arrows for "bounding box" confusing. For me, arrows refer to a Direction, so my first impression is that you're choosing the direction for a transformation, except that doesn't make sense here.
There's a really nice existing blueprint for anchor points here: https://blueprints.launchpad.net/inkscape/+spec/transformation-anchors
For the sake of the future interface consistency, I'm wondering if that arrow-less grid shouldn't be used instead, though maybe in a bigger version. A mouse-over will pop up descriptions like "Use objects' lower-left bound."
Another question is... how does this feature handle it if you want all the objects to Just touch the guide edge, but you don't rotate the objects? Basically, for an object on the upper-right, the lower-left bound should be used, while for an object on the lower-left, the upper-right bound should be used (which isn't quite an accurate way of putting it either, since the object doesn't necessarily touch the circle that way).
See what I mean? Something like "Objects to touch circle's inner edge" (or just "Touch inner edge") and "Objects to touch circle's outer edge." Then again, if anybody needs that, they could submit it as a feature proposal later.
I really look forward to the feature, rotational arrangements are a pain to do right now (I can never remember how to use that tiling interface). So thanks in advance!