Hi Mike, yes the firebrick on the sides of the Harman firebox are vertical. Not tapered to a V. The Harman coal burning products are designed to burn coal first and if you want wood.. The Tapered 'V' fireboxes are for burning wood, and if you want to struggle with it, burn some coal.
The vertical sides will help a lot with burning bituminous coal, but won't eliminate all the characteristics of Bituminous. The bitum will still swell and stick, but once broken up a few hours after loading, you shouldn't have to mess with the fire for the duration of the burn.
Harman grates are pretty aggressive too, which will help with chewing up the ash from the Bitum coal. The bitum I burned made lots of hard ash that my slider grate design wouldn't grind up. I had to clean out the clinker and ash every week or so, depending on the amount of coal burnt.
The harman hand feed boilers are a good product... You may want to look at the AHS multi fuel boilers too, they handle bitum well too. Forum member 'ktm rider' has an AHS boiler, and loves it.