This proved to be a pretty simple one, thanks to prompt responses from the majority of the relevant casinos' PR people (one of whom even did the math for us while we stayed on the line!) Here's what we learned:
MGM Resorts International: 37,103 (excludes Four Seasons and Mandarin Oriental, but includes The Mansion at MGM Grand)
Caesars Entertainment: 23,200 (approximately -- this data was delivered in rounded numbers per hotel)
Venetian/Palazzo: 7,093
Boyd Gaming: 5,302
Wynn/Encore: 4,750
Station Casinos: 4,062