This is not the "ideal" situation and I am even hesitant to comment because I am sure you will get many opinions, however.
I would insert the flange and then "mud" around it, Once the mud is dry I would drill the tile with a mason bit and insert mollies and then secure the flange down with screws into the mollies.
Once the flange is has dry mud around it and then the flange is secured into the tile, then you could install the toilet.
Thats just what I would do.
Hope This helps, I am sure there will be other opinions as well.