How can I extend an offset toilet flange when tiling?
What you are looking for is a riser also called extension flange. But considering you are able to get to the pipe below - you could just extend it to what you need, but cramped space and a crappy job - and possibly not being able to seal it properly well - could be really ...

Humor aside take a look at the links here I think you will find it to be easier.

I do believe they make spacer flanges in @ 1/2 and 3/4 inch increment.

See this link Extension Flange

There is more detailed information provided here for 4 options but extenders and spacers are straight forward and simple.