Ok, so after a few more searches online, I did manage to come up with a bit more information to add to the confusion about the architectural origins of the East Coast HM...
It seems that a number of websites are referring to the WDW Haunted Mansion as being modeled after the English Tudor style of the Hudson Valley. This may have, in fact, fueled my speculation.
http://www.hauntedmansion.info/wdwinfo.asp claims to have the straight scoop on this issue. Citing a "fact file" obtained from the WDC intranet, they support the claim that the Haunted Mansion's exterior look is similar to that of the mansions in the lower Hudson River Valley.
So, who is correct? Or, perhaps, was the final design more of a composite of both the Packer Mansion and the Hudson Valley homes?