What is the hard evidence for Beotian helmets in Roman horse? I know they're widely modelled that way, and I like the look, but is it reliable?

Which period?
PreMarian, a mix, but then they changed to gallic ones.

I don't think there is any other than Polybius's description that equites had long since adopted the Helenistic-style equipment. From what I've read the cavalry probably used a mix of Montefortino, Etrusco-Corinthian and Agen and Port types as well as the Boeotian helmets. As it was intended specifically for horseman the Boeotian was most likely used more? With every soldier providing their own armour who knows.

The Aemilius Paullus monument at Delphi was Roman horsemen depicted but alas the heads are corroded away.

I see Murphy ruled in antiquity too. But thanks anyway.