The game checks the story files when you step on the circle, to see whether you opened any of the tombs. You can trick the statue into performing the sacrifice if you temporarily use story files that say you never opened the tombs. This must be a temporary file swap, since the 'new' story files may break other quests.
in a new save, backup the story.* files, so you can put them back when the quest is done
place the story files from a save from before touching the tombs into the new save and load the game. This can be from an old save, or a new game (as in 'new' on the main menu).
If you had a -recent- save before entering the place of sacrifice you could simply reload. The temporary use of story files from an earlier point (or new game) will allow the sacrifice to take place without forcing you to redo whatever stuff you have done since you opened the tombs. This is particularly handy when such a save does not exist, so the choice is between using the temporary file swap or not completing the quest.
By placing the other set of story files in the new save and loading it, the game will think you have never opened any of the tombs. This will also effect other quests, so you need to put your original story files back at the end of the procedure.
Think of it like this; if your name (but not picture) is on list of people not to let into a bar, you can stick a fake ID in your wallet to enter, but you have to put your original driver's licence back when you leave, or at the next police check stop you are going to have problems with mismatched IDs and vehicle registration.
Clearer?
