I'll admit to not having much knowledge about the Expedition to Castle Ravenloft published adventure (Nothing dealing that closely with Strahd should even be attemptable at the published levels).
Keep in mind that the Expedition to Castle Ravenloft adventure is based upon the 1st edition Ravenloft module, before any of the Demiplanes of Dread mythos came into being. In the original adventure Strahd was a Vampire Magic-user 10 (the equivalent of a 10th level Wizard vampire... which is what he is in EtCR), and the module was designed for characters of levels 5-7 (IIRC). Expedition to Castle Ravenloft follows that module design, being built for characters of levels 6-9. I bumped it somewhat, because my players started the adventure at 9th level (Strahd has the Vampire Lord template, and has 3 added levels of Master Vampire, making him CR 18, at least in theory). Given the goals of the module (to mirror the
original Ravenloft adventure), I think they did a good job. I just have to fiddle with it because my PC's came in at a higher level.
That said, all your info on the Demiplanes of Dread has been quite helpful. I think I will make it impossible to leave Barovia, via teleportation or walking, unless you succeed on a DC 50 Will save (basically, you must roll a Natural 20, attempted once per day max). Then, when Strahd has been killed, the DC's will drop to a level-modified version of that found within the module (DC's of 23 and 21, or something like that).
The dimensional anchor idea doesn't work too well because it interferes too much with the creatures in the module (several demons who teleport, and Strahd Dimension Dooring around), and the PC's have teleported around some within Barovia already (between town and a tactical location in the castle, etc.) Not a bad idea; it just doesn't fit in this case.
Thanks for all the help, everybody!