Rincewind never needed to escapeescape the capsule since he wasn't inside of it but rather on top of it as it went over the edge of the Disc.:
The wheels slammed into it, but the only effect was to make the ship rock, Twoflower to lose his grip on the ladder and fall into the cabin, and the hatch to slam down with the horrible sound of a dozen fiddly little catches snapping into place. Rincewind dived forward and scrabbled at them, whimpering.The wheels slammed into it, but the only effect was to make the ship rock, Twoflower to lose his grip on the ladder and fall into the cabin, and the hatch to slam down with the horrible sound of a dozen fiddly little catches snapping into place. Rincewind dived forward and scrabbled at them, whimpering.
We can see this very clearly in this screencap the 2008 "Colour of Magic" mini-series:
As the ship flies over the edge, Rincewind is chucked off the ship onto one of the trees in the rimfall. With no way of getting back up, eventually he decides to let go.
In the sequelAs to how he managed to get back onto the Color of Magic (entitled The Light Fantastic)disc, in the sequel novel The Light Fantastic we learn that the Seven Ultimate Spells detected that the "Eighth Spell" lodged in Rincewind's head was about to leave the Disc and contrived to cast an enormous 'change spell' to teleport him somewhere closer to the centre.
“It’s a Change spell,” said Trymon. “The whole world is being changed.“The whole world is being changed.”
Death also makes it abundantly clear what the spells in the Octavo were up to:
THATTHAT? OHOH, THAT WAS JUST A READJUSTMENT OF REALITY. I UNDERSTAND THE OCTAVO WAS ANXIOUS NOT TO LOSE THE EIGHTH SPELL.THAT WAS JUST A READJUSTMENT OF REALITY. IT WAS DROPPING OFF THE DISC, APPARENTLY UNDERSTAND THE OCTAVO WAS ANXIOUS NOT TO LOSE THE EIGHTH SPELL. IT WAS DROPPING OFF THE DISC, APPARENTLY.