Mark J put in the hard work to get it set up. He'd had a bit of a false start trying to translate BBB directly to the online environment, with free movement and with a token for every 1" base. This made it very cumbersome to move multi-base units and difficult to see the individual tokens.
For the second and very successful iteration, he adopted an idea I'd toyed with a couple of years ago, effectively converting it into a hexes-and-counters boardgame. All the measurements in BBB are in 3" increments, so it fits naturally into 3" hexes. We represented each unit with a single token, with its strength and formation noted on it (these can be easily amended during the game).
We did encounter some interesting questions: how to define the front arc, how best to display the necessary info, how to define whether a unit is supported by another, etc. But nothing we couldn't resolve. The trickiest problems arose from the map itself, which wasn't designed for hexes and didn't always match them well, hence issues of whether a unit was or wasn't in a wood, or had or hadn't crossed a stream. Again, nothing irresolvable, certainly between friendly opponents with the assistance of an umpire.
In a 3-hour session on Monday night (the usual OWS club hours) we managed to get through most of the First Battle of Bull Run. When you allow for time spent resolving the above translation questions, and time spent getting familiar with the technology and how best to play the game with it, that makes a BBB battle on Roll20 very feasible in a sensible amount of time.
The biggest chore every time is going to be the set-up. But even that should get much quicker and easier once the best way of doing it has been worked out. Mark has already made good progress refining it further since our test session. I foresee a lot more BBB gaming on Roll20 in the future.