Is there any historical precedent in 18th Century American timber-framing for an occurance of an English tying joint without an integral continuous wall plate? Has anyone ever encountered or documented such a joint in an 18th Century frame that incorporates an interrupted plate and, if so, can the tying joint still be considered an "English tying joint"?

My answer would be no, it is not possible to have a interupted plate and a English Tying Joint, 18th century or otherwise.
A English Tying Joint is dependent on the Tie Beam having a dovetail cut into the underside, joining it to the top of the Top Plate and the Top Plate joined to the top of the post. If you have a interupted top plate it would not be joined to the top of the post but rather joined into the side of the post, away from the Tie Beam joinery.
It is the integral joinery of the Tie Beam, Top Plate and Post that make it an English Tying Joint.

steve chappell's book "timberframer's workshop" has a "modified english tie joint" which uses a non-continuous plate...page 87
I realize this is not an 1800's book, but it does look possible to use the joint without continuous plates....though I admit it's hard to see how the dovetail is effective when used this way.
-mark