Thanks for the thoughts. I hadn't tried using the explicit join eyntax, but I just tried it and it generalted the same "Invalid Identifier" error that the implicit syntax did.
I have tried similar queries as commands, and they do work. As I said above, I think the problem is that, when the SQL Expression is saved, it isn't done in the context of the larger report, so the database doesn't have the whole picture. It doesn't have the reference to the included report that's in the main report, so it sees it as an invalid identifier. I've also tried including the table in the FROM clause of the SQL Expression, and it does save without the error, but when the report runs the expression doesn't link to the main report the way it should - it runs with a separate instance of the table.