The mortar and set up you have I am familiar with
At a slow speed, in theory I think I could do it!
If its used in the direct role, piece of cake!
In the indirect fire role, sight it in, and if the movement is not so fast and you take into account the movement, and more or less lead the target you could be on target, remember a mortar is an area weapon, and if the rounds land close to the target it counts just like horseshoes and handgrenades. A moving platform would possibly act as a traverse fire mission, where the rounds land more or less in the line of travel, going down river, keeping in mind that a minor distance on the sight will be a greater distance down range, so, firing at a rapid but steady and unhuried pace could give a similiar effect as a traverse mission. Again, this is theory and oh to be on a gunline again to test it, as it does sound interesting.