as a heads up since about several months ago, maybe 2012 April, ex-TPE I-tickets have changed most of its stopover rules such that you are only allowed HK stop-over on OUTBOUND not inbound. This is exactly to discourage many guys like me and my colleague who used last-I-class segment of the ticket to fly to TPE on commencement of next journey.

With new rule they force you to purchase a one-way ticket to TPE to commence your next travel.

The work-around is, after you returned from your destination on the ex-TPE ticket, fly to TPE on last coupon but return to HK on the first-segment of next ex-TPE I-class ticket, then have stop-over. Of course this assumes you are frequent traveller of same route, as most I-fares have validity of 3mths or so.