Here's a closer look at the inscription. The glyphs say that the prisoner was the vassal of a Palenque lord called "The Ballplayer". This is probably a reference to the son of Pakal the Great, K'inich Kan B'ahlam II.
The "Ballplayer" glyph is the second from the bottom on the right. Beneath it is a fragment of the Palenque emblem glyph, in its "wavy-bone" form. Photo: Joel Skidmore.