I think it could work, because you could split the minutes at C, and give Holmes a few extra minutes at PF with a shooting big alongside him.
Poeltl is an average starting center and a great back up center, so I think it serves the Spurs well to put him in the position of a 24+ mpg bench player. They'd rarely if ever be on the floor together. But another aspect of Holmes is his steadily improving shooting-- the one year he averaged over 1 3ptFGA per game, he shot 35%, and his numbers indicate he's probably ready to add that back to his game. His FG% over his career shows impressive nonstop improvement (.514, .558, .560, .608, .648, .688) as does his FT% (.689, .699, .661, .731, .788, .844).