Glacier, I asked Transport Canada this exact question about 2 years ago. The advice I was given from Ottawa is that there is no need to maintain anything in Canada, as long as you maintain a current IFR licence whilst in HK, and you had an ATPL before you left. You can PM and I can give you the contacts at TC, or you can just ask them yourself.
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Quoting Transport Canada:
If you choose to maintain your Category 1 Medical Certificate and complete a Canadian IFR ride (maintain a Group 1) every two years, then you are correct in that you will maintain your Canadian ATPL-Aeroplane and all the privileges attached to it ( CAR 401.34 (Privileges), CAR 401.48 (Validity of instrument rating)).
In your particular situation as an SO, if you let your IFR lapse then we apply CAR 421.49(4)(which applies to Canadian pilots flying for foreign Commercial operators outside Canada);
You can let your IFR rating lapse for up to 24 months prior to renewal, without having to re-write the INRAT Examination if: you are in possession of a foreign Commercial Pilot Licence or ATPL (Hong Kong), have a valid foreign Instrument Rating (Hong Kong), and successfully complete a foreign instrument flight test (Hong Kong) within 12 months of application for the renewal of the instrument rating, while working for hire or reward in a Commercial Operation outside of Canada (Cathay Pacific).
Your ATPL doesn't "lapse" as such. In order to exercise the privileges of the ATPL you must have a valid Category 1 Medical Certificate and a Group 1 Instrument Rating. Maintaining a Category 1 Medical Certificate maintains your ATPL ( your Medical Certificate maintains the validity of your licence). The Group 1 Instrument Rating affords you the privileges of the ATPL. Even if you did not maintain a Category 1 Medical Certificate and Group 1 Instrument Rating, you still have a Canadian ATPL document, however you cannot exercise any of the privileges attached to an ATPL. You would not ever have to re-write the ATPL examinations. As soon as you do a Category 1 Medical, your ATPL becomes valid with Commercial privileges. Then, the Group 1 Instrument Rating would reinstate the ATPL privileges.
I would recommend at this point in time that you maintain your Canadian Category 1 Medical Certificate. You could do a Canadian IFR ride every two years, or not. As long as you can produce for us proof that you are doing an annual IFR ride with Cathay Pacific at least once a year, we would accept that as you being qualified for not having to re-write the INRAT exam, whenever you decided to renew your Canadian Instrument Rating.