How can I tile over a floor where hardwood is attached directly to joists?
Not doable at all.

Old houses have 3/4'' thick wood planks as subfloor, not 8x4 sheets of plywood like today's houses, so I would say that it's ok to consider your hardwood floor the subfloor. That's if you're putting another wood floor on top.

However, for the purpose of tiling you need an additional layer of plywood to reduce deflection that will cause the grout to crack eventually. And on top of that you would install your cement board. The cement board itself provides no structural support and is only used as a decoupling between a wood subfloor and the tiles, so the lateral movement of the wood is not going to cause the tiles to pull up. You can always use a durock membrane to reduce thickness, but you still need a floor that will not deflect.

If you really want to do this right you would take out the old floor down to the joist and install some OSB and then do what you want...