If so, what was his storyline to be?
I understand that Castellano may have been skittish on Clemenza betraying Michael.

I'm not aware if there is any, at least none I saw, the earliest one I recall still had Pentangeli in it but some variations. I imagine it would have been roughly identical, just with Clemenza in place of Pentangli, but played out exactly the same.

I'm sure there's a script somewhere with Clemenza in it.

The reasons given for Castellano not appearing in II are that he wanted too much money, and that he insisted that his girlfriend (who appeared as his wife in GF) wanted control over his scripted lines.