Address 03AF and 0356 will change the animation frames of P1 and P2 respectively... but only during their "throwing the ball in the air" pose, though.
Every object loaded in VRAM can be displayed, including regional text.
It only loads the current directions for either player though, so you'll need to do this in both sides to get a full rip of a character.
(or you can force both players to be the same character via addresses 3CA and 371)

Disabling the background layer in BizHawk produces artifacts next to the sprites.
As such, keeping the background layer enabled but recolouring addresses 1238 to 1240 to one shade (hex 5050 for a lovely purple) worked nicely.

The player sprites are stored in three chunks -- head, torso, and legs, with a shadow applied underneath.
Since these are screenshot rips, the shadow is baked in. I figure blanking the shadow in the ROM or VRAM would allow you to rip them again without it...!



343	p2 x-pos
356	p2 animation frame
371	p2 character

39C	p1 x-pos
3af	p1 animation frame?
3CA	p1 character

401	court

404	camera?
494	camera?

1238~1240	bg palette