On the desktop website, a workaround used to be to first select "Book a ticket" (cash fare), choose "Multi-city" and set up your itinerary, then as a last step before selecting "Search flights" choose the "Book with miles" option. The search would then return the price of the multi-city segment in miles. I used this workaround as recently as 6 months ago.

However, it seems they've patched this in their backend system – and the workaround no longer works. Does anyone have experience with calling FB and getting an agent to price up a multi-city itinerary?