Hi everyone. Been lurking for ages here, thought I'd post on this subject
I've been toying with using the Dominions background for a P&P or PBeM game for a while now. It happened when I realised that Late Age Ulm is very very close to the unnofficial setting that a lot of my old RPG games were set in. Although there was never any official connection between any of my games there were always similarities and common themes running through them, ideas and themes that are prevalent in Late Age Ulm. So I started comparing other games I'd run and idea's I'd had and found they could all fit very nicey into the Dominions world, with a couple adjustments here and there. I think the Dominions setting would make a fantastic basis for campaigning. I really love the mix of fantasy, mythology and history, something a lot of RPing settings don't get right.
My advice for using the setting as a backdrop would be to focus on a small part of the Dominions lore. Rather than throwing in all the nations of the game and flooding players with information just focus on one or two of them, and a couple others that thematically fit your campaign as neighbouring realms.