The Qi charging will only work if the charging pad is in contact with the ‘bottom’ of the Roam.  So if that can be done while the Roam is lying horizontally, then it would charge.  I would assume that is no problem with the Roam specific charger since it is sized for the Roam and magnetically attached.  Other Qi chargers might require more creativity to get it to work.

Yes, I can confirm that. With the original Sonos charger you can lay the speaker down as the pad is magnetic. If you use any other Qi charger it does only work with the speaker standing.