Can I install a toilet flange in a shortened (sawed off) elbow fitting?
Were this my project I'd cut off the fitting flush with the floor and find out exactly how close to normally the flange fits. The inside of elbow should be the same size as the inside of the pipe. If possible, I'd make slight adjustments to the flange by sanding or grinding, then glue it in using primer and PVC adhesive.

I'd then be sure to securely anchor the flange to the concrete to minimize stress on the joint by the toilet mounting bolts. 

This solution won't likely meet applicable codes, and will probably generate a few downvotes from sticklers who think there's only one right way to do things, but if done well will not create problems.  If so much material needs to be removed from the flange as to leave a poor connection or structural weakness, or if the curvature of the elbow prevents a good fit, you'll need to remove the concrete and start fresh.