There are two things with tce-load I'd call odd:
First, you can run
tce-load -w -i extension.tczto download and install an extension, but
tce-load -i extension.tczfails, even if the extension has been downloaded.
I've attached a patch that prepends $TCEDIR/optional as a second try, if the extension was not found in the first place.
Second, if you try to install locally an extension that depends on compiletc.tcz, it will fail. To reproduce download only the qt-creator.tcz creator package and install it afterwards locally or via ondemand.