I haven't had a chance to try it out, yet, but the SetContentListSubset() method appears to be what you're looking for. I'm pretty sure you'd use that to load your initial page, then call it again for additional pages based on the isListItemFocused() event's Index (i.e, load page one with items 1-20 initially, then load page two with items 21-40 when isListItemFocused() >= 15).
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