Hi,
This just an idea that would work but only do it if you don’t plan to remove the tiles and go back to timber floors in the future.
Using a circular saw cut the hardwood floor out where you plan to tile the floor and replace the timber with Cement Boards.
The cement boards come in different thickness, so when you remove the timber you will be able to work out what is the best thickness to use.
These boards are a perfect place for tiling over and they can be used for both wall and floor applications. Below is a site that shows you picture of the cement boards being use for floor tiling.
http://www.unifixinc.com/products/permabase.htm
regards from Noelene