Try for an car exhaust manifold paint e.g. Rutland Premium 1400 Degree F Hi-Temp Brush-On Paint - there are others - only 100 deg better than 650C but it is a start.

A search for manifold paint on Amazon brings up loads of possibilities including some claiming 800°C

Is the paint for appearance or to prevent corrosion?
If it's to prolong the life of things like fire bars thatcan get white hot then I'd have thought it's a doomed attempt.
I know a domestic grate soon burns away if you burn top grade welsh steam coal, so presumably steam engines either use better quality iron or else the components are regarded as expendable and replaced periodically.

Thanks for all the thoughts and suggestions.  It was just for appearances, not as protection.

I'll be the first to hold may hands up and say this is a quintessential first world problem!  But it's the firebricks in my stove that I find really ugly, being the pale yellow-ish colour that they are.  Especially when the fire is lit and bright and you can really see them.  So I was hoping to paint them so they disappear more.  I'll also be giving the baffle plate a once over at some point and was planning to finish with some paint on that too.  

No big problem though if no paints would be suitable.  

Cheers