You’ll find rigid ’29er’ forks at around 470mm A-C; I don’t recall seeing anything more than that … There’s a decent choice around in that kinda A-C length too, Kona PII’s, Salsa CroMoto Grande, or the Fargo (V2) fork if you want some interesting mounting options.
This is almost certainly of no use at all as an observation, as the frame geo’s will be very different, but I run a set of 29er PII’s on my old (1992) Kona, with a big 26″ tyre, and it’s a *much* better ride now than it ever was on either it’s original PII’s (~440mm), or the 100mm Manitou’s it had for a while. Granted it’s pretty long now, so it’s not for tight, ‘technical’ kinda riding, (‘tho it never was) but I find it perfect for just hacking about on.