Interestingly enough, when I researched this with my Kubota owners manuals a while ago, I got nowhere too. The tractor manual provides tire pressures with no mention of adjustments for FEL work. The manual for the FEL provides the same tire pressures as the tractor manual, again with no mention of adjustments for the FEL being loaded. I have two Kubota tractors and four owners manuals, same result in both cases.
My thought process suggests to me that an increase in front tire pressure would be warranted when the FEL is installed, since the load on the front tires is increased, at least to the point the rear counter weight isn’t lifting the front of the tractor off the ground. Myself, I do increase the front tire pressure about 5 psi when the FEL is installed.