Going along with this idea, bikes with odd, non-circular seattubes could indeed have two threaded holes, say rivnuts or similar installed on the seattube and the manufacturer's could provide two or more replicable bolt-on tabs with different dimensions to cover a range of large chainrings. The main problem with the current "braze-on" tabs is they are permanently brazed or riveted to the frame and, therefore, not sufficiently adjustable.