Securing a toilet flange on top of ceramic tile
I see very many toilets fixed down with only silicone sealant and blank plastic plugs in the fastening holes. One was in a home owned by a guy who weighed in at 400lbs (30 stone or 190kgs)...

I've also struggled to remove toilets fixed in such a way! 

I wonder if this may be a national issue though, as I'm not quite sure what you mean when you say toilet 'flange'. We call them a toilet 'pan' and they have one or two holes each side at the bottom for screws, often going into a plastic bracket inside the base.