Accounts before and shortly after 2000 do not discuss a foot-washing. Some that I just pulled off the shelf to check include: Laurie Dennett (late 1980s), Jack Hitt (early 1990s), Hape Kerkeling (2001), Tim Moore (2004). I'm afraid this does cast some doubt on the reliability of her account. Had their feet been washed in the cathedral, I'm sure these authors would have mentioned it. (I can't speak to my own experience in 1989 as I didn't seek a compostela that pilgrimage.)
While Lavacolla was certainly a historic washing point, it isn't foot-washing I'd always heard it associated with. ;-)
The foot washing place I'm familiar with is somewhat earlier in the Camino, at the Saint Nicholas albergue run by the Italians. I'm not sure how long they've been there doing that, though. Being charitable, perhaps Ms. MacLaine experienced it there and moved it to Santiago in her book as literary license for greater impact?