> From what I understood the HBr/H2O2 was used to generate
> in situ bromine,
Exactly.
> which I think NaBr/H2O2 in HOAc could do as well.
I tried it by reacting NaBr, AcOH and 30% H2O2. No reaction was visible, no colour change. When ~30% battery acid was added bromine was produced instantly.
The reason why H2O2 should be added slowly IMHO is the following:
2x HBr will produce Br2, which brominates the aromatic nucleus forming another molecule of HBr, which then has to be re-oxidised into Br2. By adding all the H2O2 at once, you will (right at the beginning of the reaction) have only half the required amount of Br2 for complete halogenation present and half of the H2O2 will be left over unreacted waiting for more HBr to show up. This excess H2O2 might do bad things to your stuff, or over-oxidise the Br2, or partly decompose. All side reactions which will potentially lower your yield.
If you wanted to add all the H2O2 at once I'd use at least two moles of NaBr per mole of amine plus the same amount of a strong inorganic acid, to ensure complete conversion of the bromide into Br2 and limit side reactions.
Never heard of NaHCO3 being successfully used in nitrostyrene preparation. I guess it will be pretty insoluble in the nitroalkane, and therefore might not react.
Do some microscale test to find out if it will work, but I'd rather use one of the proven catalysts here.