The answer is, "yes". Nordstrand will
produce pickups with custom spacings. Last year I bought a set of Nordstrand Dual-Coils, with the polepieces custom-spaced for a Carvin LB75 (standard string spacing, not the wide string spacing), and they fit beautifully and sound great.
As for custom casing sizes, I couldn't say...
P.S. Yes, Carey Nordstrand is an active member of TalkBass.