I have a friend with a third function grapple on his L3800, works great. I am not sure that it would run a large brush or other high flow skid steer implements, but for a grapple, not a problem. I would recommend a setup that adds the third function actuating switch like a excavator, IE is part of the handle as in a rocker switch rather than another manualy operated valve so you can still be tilting and rasing the bucket as you lower the grapple. You only have one hand to use on the hydraulic controls and it is hard to manually move two levers at once!
So I do not agree with your dealer. Just take a look at the pump output in volume and the pressure rating on the pressure relief valve and size teh grapple cylinder(s) accordingly.
It does seem that some dealers look down upon these "economy" tractors as somehow being inferior, and as they are admitantly built to a certain price point and do not have all the bells and whistles as the Grand L series, they are certainly capapble machines within there design limits.
Kubota even resticts these tractors in some respects as to a front dozer blade, mid PTO and such. Not that they are capable, but Kubota is not stupid and if they can "steer" buyers towards a higher priced better profit tractor they will by offering a good equipment list for the economy tractors, but not all options... saying something like this.... "while this L3800 is a great tractor, for that paticular option sir, you must get the Grand L....."
Oh, and I would get a standalone grapple setup, not one that attches to your bucket if possible as it makes lifting long items up easier as you do not have the sides of the factory bucket to get in the way.
David