When I finish my current Guard projects I plan to make a Vostroyan army that includes some FW heavy mortars. I like the concept, but the mortars are a little difficult to use in regular games so I considered using them as counts-as Griffons instead.
I was thinking about mounting a heavy mortar along with a heavy bolter equipped Centaur or transport Scylla on a large rectangular base slightly longer than a Chimera.

What do you think about fielding these in pickup games? I don't really plan on using Centaurs or Artillery (except a possibility of thudd guns in Apoc), so confusion should be minimal.
By the way, which would look better: Centaurs or Scyllas?