There's no easy way to do this because of the design of the boilers. You got a door and a box. Gravity works. the farther away you try to stick this huge chunk of wood, the harder it is to handle.
If they were designed to be chute fed, it would be easier. The big industrial boiler at one shop I worked at used a chute and conveyor system, taking advantage of gravity, all the wood fell down into the boiler, it wasn't shoved in sideways.
I don't have an OWB, but if I did and wanted to burn big uglies, stuff hard to split or weird o shaped, etc, I would rig a tripod support then a long lever on that with tongs on the other end. Swing it over and tong up the chunk, swing it over and into the opening as far as it will swing in (you are using leverage so this is big, see?), lower it a little to release pressure on the tongs, then push it the rest of the way in.
Or like that there. You are basically trying to come up with a combo forklift for picking it up then something that will move the piece forward into the boiler then withdraw that piece after your cargo delivery, whatever that piece is that is moving it. Rather complex to be made cheaply I would think.
If you can make a hydraulic lift table, which is also portable, meaning it will roll in front of the door then away or when it is not being used, this could be easier to do. You have to be able to move it in place and away easy though, it can't just be heavy and live in front of the door all the time. A lift table is rather straight forward in design, that shouldn't be hard. Roll/flop the big chunk in, lift it up, stop it at the correct height, then perhaps a light forward rolling table, enough to get it in there and then just manhandle push it in. You *could* add a second pusher from hydraulics, now you have doubled the complexity and expense. Have to make it so the big chunk can't fall the wrong way and squish your customer...
Red neckerson fastest and easiest, perhaps, an engine hoist type arrangement, or like those pickup lift arms you can buy already, and a kid's slide aimed at the door. Chunk goes up, then ya slide 'er in.
Of course, being a nerd, I would much rather have something like this..suitable for loading wood boilers and other interesting pursuits