Greg Patent wrote what may be the definitive article on the history of Boston Cream Pie for the journal Gastronomica back in 2001. He had it on his website, but it looks like the link is messed up again. I'll drop him a note to see if he can fix it again.
I had a series of emails with Greg a few years back, I was convinced (and he later confirmed) that the original icing on a Boston Cream pie was made using confectioner's fondant. (This is NOT the same thing as cake decorating fondant.) Almost nobody does it that way anymore, though, most use a chocolate ganache, whether it is semisweet or more of a milk chocolate ganache seems to vary depending on what part of the country you're in.
When I took my chocolate academy course last fall, other than the instructor I was the only one there who had ever made confectioner's fondant. It's made sort of like an unflavored fudge, but then is creamed on a marble surface until it becomes crumbly. Then you store it in a sealed jar and after a few days it softens and becomes almost like silly putty. It will last several weeks at that point.