If you don't end up using the bandgap as a direct ADC reference, if you are willing to do two conversions you can use it as an indirect reference, as spelled out in AN3031 or my post:
On-chip temperature Calculations
As noted in there, the bandgap stability is, however, 'not great', with an 80mV drift band (Vtdrift) across temp, which is > +/-3%.