Do I need a BoB when replacing a parallel port connection with a UC300 or UC400?

[A_Camera]

Okay, I take it back. That's a new configuration for me. But the advice still stands - you can connect the UCx00 easily enough (though options to incorporate into the physical enclosure are a bit more limited). If that big PSU is a 24V DC powersupply for the board then why not slave the power from that?

Well, that's not entirely true... The UC300 and 400 needs some sort of a BoB, because you need to split the contacts. Doing that without any BoB is possible, and as you said, most drivers have optical isolators anyway, for for that, you don't need a BoB but you definitely need some way of splitting the signals and lead them to the right contact on each driver. So "easy" is a very wide word. With a BoB it is easy, but without it is not so easy.