Can you generate a pseudo supply from the range of supply you have.
I am talking about an LDO . An LDO would also need a reference but that need not be a BGR it could be cruder say voltage drop across diode chain. Just see to it that for higher temperature your requirments are met.
Hope it could help.
A very good idea (buck converter) --> you have a cleaner supply voltage for the bandgap reference and I also think power dissipation is much better than a simple 6-40V bandgap reference (you have many tails, drawing current *!!! max. 40 V --> )