Well, after a long winter and finally getting a chance to work on the boat this spring I've finally gotten rid of the dreaded belt chirp. The crank is definitely walking a little more than it should, but I'm not too concerned about it since a little extra clearance in the thrust bearings isn't going to have any immediate detrimental effect.
I ended up putting 4 small washers behind the waterpump pulley to extend the pulley out/forward about 0.5mm. Just enough to bring it in line with the crank pulley and prevent the belt from walking sideways on the tensioner.
I'm still not sure why it only seemed to chirp when the trim was in/down, but after getting it in the water this past weekend and getting some time behind the wheel all is nice and quiet. Not much of a solution, I know, but I just couldn't leave this thread open-ended. Now on to the next project; figuring out why my steering suffers from morning sickness...