I don't know, is this something that can be specified with the data
item? (Rather than the style language.)
just to reiterate: locators are specified for each _citation_ (rather than in the item data). E.g. I could be citing an entire chapter (ch. 3) in a book at one point, but a specific page number or range at another point. Those would be different locators with locator labels and can be handled elegantly by CSL, which covers about a dozen different labels. But they serve a different purpose from pagination.
@Bruce:Could you at least suggest a general idea of what type of solution you'd prefer? "I don't think that's a very good solution" isn't particularly helpful ;), especially when your main reason seems to be a situation that I'll doubt you'll be able to find any example for.