Here's how I got things back to normal: while booted under X, I opened the trash (successfully), then double-clicked on the "Desktop (System 9)" folder and created a new folder. I then Selected All files in the OS X trash and dragged them to this new OS 9 folder.
The move was successful, and since then the OS X trash has behaved normally. The next time I booted under OS 9, I dragged the new folder (filled with OS X trash) to 9's Trash and successfully emptied it (using Option/Empty Trash, as some files were locked). I regained the appropriate amount of disk space, and the problem has not recurred as of this posting.
I don't know, but suspect, what happened: I think I must have put some files that originated under OS 9 into the OS X trash.

