Chas Roberts custom track

-rear brake routing. Internal or external? Any benefits or just aesthetic? Rossssss's Nag is external and it doesn't detract attention from the frame.. Can't decide though.
I'd go for internal. does look good and clean (aero benefits?)

-phils or custom made matt black mack hubs w a pink stripe. High or low flange?
low flange if you're gonna have low profile rims. tends to look a bit odd otherwise IMO

-cranks: prob 75s or omniums? Thinking of 75s atm..
Good choice. Omniums would look too slave?

-stay classic with a 1" steerer or go 1 1/8"? Overall it probably depends on the other tubing to make sure it suits overall.
Like you said, aesthetics matter. Fat heattube on thin tubes may look odd. You could shim the 1" of course, but you're looking at A-head stems no?

IMO, IMO, IMHO!!!