Thought I'd provide an update. After trying just about every conceivable technique, I still can't get the roll paper adapter on the Epson p900 to load Lyve canvas. I've tried using a 6 inch wide paper lead attachment, which didn't work. I may try an even wider attachment in the future, as per Dan Berg's suggestion.
Fortunately I have had success cutting the roll down into sheets, then attaching a paper lead, and loading through the standard back print loader. Not an efficient technique, but at least it works. Judging by the lack of replies to my initial post, I guess there's not too many folks using the p900 roll paper attachment for canvas.
I'm guessing that since I'm using a non-Epson canvas, I'm probably stuck with the adapter, which may prove to be a waste of money for canvas printing; live and learn.