Francis is an expert at putting words (preaching empathy and support for the poor) into dramatic action ("Mercy Friday" visits and a thousand other actions), and in this he his following the example of scripture (OT prophets regularly were prompted by the Holy Spirit to dramatic gestures; Jesus regularly did the same, especially in the Gospel of John which is arguably structured around seven dramatic "signs"). 

John Allen over at Crux (https://cruxnow.com/analysis/2017/04/02/time-dramatic-flourish-u-s-bishops-immigration/), referencing this sort of papal action, praised a similar symbolic action of a few American bishops, led by Sean O'Malley, in 2014. Eight bishops expressed their support for immigration reform by celebrating mass at the border in Nogales AZ, including the gesture to distributing communion through the slates in the border to would-be Mexicans immigrants on the other side.

Allen suggests an expanded repeat performance, a perfect "sign" in this age of Trump's wall.