I wouldn't try the spacer ring method if I were you; too many extra variables that could go wrong. And you would have to countersink whatever fastener you would use to try and secure the spacer down to the subfloor (I am assuming concrete slab) and most likely crack it. I assume by "hard tile" you mean porcelain tile, which is pretty difficult to crack by drilling unless you aim to break it!
But before you consider all of that, if I understand what you said, your pipe is flush with the subfloor now, and there will be a gap of 3/4" between the top of the pipe and the finished floor...if that is the case..that's a sizeable gap...and you should consider just installing the flange flush with the subfloor and having the tiler install around it! You can always use spacer rings or double the wax rings to balance it out afterwards...not the best method, but certainly one worth considering in your case.
Maybe someone else has a better idea?