Hi Andrew,
I'm not sure how to get to the placeholder properties dialog box.
As for the error, I assume that would be because of the version difference in the FCKEditor being used. The 6.3.15 I found to be version 2.3.2 and the 6.4.0 trunk one version 2.6.3.
If that is the case, you will probably be foreced to use the trunk (newer) version of the FCKeditor, to support your plugin.
This one will be hard to write an issue for on 6.3.16, as it is already fixed in 6.4, so I think you must find some workaround that works for you. Personally I would be really tempted to go back to the version 6.4 fckeditor folder and then make an attempt to "patch" the fckeditor to work around this, as this will hopefully be just a temporary workaround until you upgrade to version 6.4.
If you are interested, you can test and see if this works for you (remember, please go back to the version 6.4 fckeditor folder first). Then find the file fckeditorcode_ie.js and edit it. Search for "oEditor.UpdateLinkedField() there should be only one occurrance. Place a Try/Catch block around it so the call should be:
try { oEditor.UpdateLinkedField() ;} catch(e) {}
This may of course affect something within the FCKeditor, but quick tests didn't show anything that didn't work. And, what matters the most, the error seems to be gone... or at least it didn't show in IE8 any more, and believe me, it did before.
Hope this will be of some help,
Palli