Hey Max,
I too think this is probably way to much deflection for something as rigid as clay tile. Not being a fan of OSB in general I would probably stiffen the floor with an interlocking jointed wood floor if possible (yes it is more work...but stiffer) and this is pursuming that the frame work can take the added weight.
I have never seen tile suffer well any deflective attachment surface as you just described.
Now, on a positive note to your idea...no guarantees here...I can share a few proprietary concoctions that my mother used and I have experimented with that give good results and may just work with this much deflections...You are going to have to make your own test panels and do a bit of expermenting.
Both of these either used "sawdust" as a "matrix filler" or "cork granules."
http://www.advancecork.com/granules.html
http://www.corkspirit.com/en/arts-and-crafts/natural-cork-grain-cork-powder-45
Formula 1:
part dry lime plaster
casin
egg white
flax oil
The proportions you will have to work out in the sampling but this mixed with the cork granules can create a workable grout.
Formula 2:
Natural latex rubber adhesive mixed with the desired level of cork grout to create the filler/grout and bedding agent.
https://www.google.com/search?q=liquid+rubber+mastic&oq=liquid+rubber+mastic&aqs=chrome..69i57j0l5.5100j0j7&sourceid=chrome&es_sm=122&ie=UTF-8&qscrl=1#qscrl=1&q=natural+rubber+latex+adhesive&revid=1118824083
http://www.chemionics.com/natlatex.html
http://www.usadhesive.com/naturalrubber.aspx
http://www.ifscos.com/products/natural_rubber_latex.html
As a disclaimer, the companies that make some of these latex rubbers are not always the "greenist."
Let me know if I can be of further help...
Regards,
j