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I think that's probably the geometrical limit. 10mm for the guard and some clearance, >10mm for the bolt and crown, and a bead seat of 311mm leaves you 36mm to get the tyre in on a 367mm fork. Guess you could shave a couple of mm off if you bolt it in from underneath with some kind of countersunk bolt.
@PhilDAS I though 380mm was road A2C! Seems like all the endurance bikes have gone to 397mm nowadays
The fork is usually always the problem if you want fenders AND 28s, aftermarket models anyway.
Have not researched OEM forks, but I guess there could be some options there.
Maybe we need to do a group order, I bet we could come close to minimum requirements for a small batch run for something custom.