I was under the impression this was entirely Probert's idea. Albeit offered at a point where the film was really struggling to come up with an ending, so in answer to the question, I don't think there was any "proper" script involving this ending, it was just offered up by someone involved in the production via the visual medium he knew best when everyone else taking part was trying to come up with an ending as well.
As I understood it, his thinking was the Klingon ships would be the same ones eaten up by V'ger at the start of the film, released by the probe as a result of... Whatever switched off the main plot (as yet undecided when he pitched this).
So the klingons would have actually had good reason to be on the offensive- from their POV they'd have gone from being in their own space under attack from a blue cloud to being in orbit of the home planet of their deadliest enemy without explanation.
Whilst a nice idea, it's easy to see why it never got any further. It would have added on an extra sequence after V'Ger was dealt with, that didn't really have anything to do with that main plot.
Plus it would have been odd to have the Klingons survive their seeming death at the start.. only to come back and go into battle with the split Enterprise. If our heroes had destroyed them... it makes the Klingon ships surviving being absorbed seem a bit pointless.If the Klingons live.. it makes any battle sequence even more pointless padding to the end of the film.