Would he consider adding several of the unissued tracks to the 1958-62 Selects that he's planning?
I don't think so, for two reasons.
He spoke of "fitting them" into three Selects, which suggests that he realizes that with all the recordings they did in that period, it might be a snug fit to include all of them.
His statement that he doesn't think the unissued stuff is deserving of release.
Also, other than the Hill Select, haven't the Selects just concentrated on original albums only and not considered alternates or unissued tunes? I think he wants to stick with that approach, and leave alternates/unissued stuff for the full Mosaic boxes.