> a vmod for the Peg2 that would lower the voltage supplied to the AGP slot
> [...] would probably entail replacing the AGP slot connector with a 4x/8x part

Yes, absolutely.

> If you could find a PCI HD 5450 (or was it 5470)

It's 5450.

> that would be an option.

Although currently no overlay with MorphOS.

> I have never seen one actually offered for sale.

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=12232&forum=9&start=37
(Additionally, eBay Germany has a used one (CGA-5452PLI) for 130 EUR (146 USD).)

> if you really want a G4

He does have a Pegasos II G4. It's what this thread is about ;-)