Thank you all. This is very interesting information.
I can really see how Portal: Three Kingdoms might inadvertently have fallen victim to the minimum 11 cards per booster rule.
Would it perhaps be possible to loosen this general rule to, say, require a minimum of 8 cards instead? That way other expansion set combinations could potentially be added booster drafts in the future.
For example, my IRL playgroup keeps a paper draft set cube, from which we draft boosters from the expansion sets of Mirrodin, Antiquities, Mirrodin (so, 15+8+15 cards for a total of 38 cards per player). This set combination is quite a blast to play.
I see that there's already a Fourth Edition, Chronicles, Chronicles (15+12+12) draft setup active in Adventure Mode, so a draft setup with a single 8 card booster in it would not necessarily leave players with too few playables total in their draft card pools, compared to what's already there.
(On a side note, I would also love to one day see a "Revised, The Dark, Revised" booster draft in Old Border mode. Or anything with Fallen Empires for that matter)
Alternatively, do any of you know if Forge can be set to run draft games with 4 or even 5 boosters per player instead of the normal 3?
That could be another way of bringing Portal: Three Kingdoms back into the draft set mix while keeping it at 10 cards per booster.
Alternatively, you could also simply ignore the original paper version booster size and instead make PTK in Adventure mode come in artificial booster sizes of 15 cards (1 rare, 3 uncommons, 11 commons), similar to most other sets.