It's impossible to circumvent LIFO on the master seniority list without significant litigation and a huge quagmire (and the litigation costs--in which you'll ultimately lose the case--will be extensive for the company while in some jurisdictions 'free' towards the plaintiffs). While the enforceability in HKG might be dubious elsewhere it would not be and you can't use 'tricks' to try to get around that in that all that information would be discoverable. Nor could you partially circumvent this by applying LIFO in one area and not in another because that would affect everyone (i.e. a partial LIFO only on the base but not in HKG would still affect those on a base so it'd still wind up in arbitration/court). There's simply no way around the provision.
SO
You're gonna need a voluntary scheme to comply with the contracts and one that attracts enough people to take it. That's really the only legal avenue before the reverse-seniority axe.
The break point (i.e. the point it'd be financially beneficial for retirement and permanent separation vs. simple layoff provisions) for most contracts is two years for a clean break (given the pay protection, AL, sick days, etc.). Medical in countries for a year or so might need to be added. This is similar to many (developed nations) scheme; I'd probably look towards Southwest for a model.
You might add a one year voluntary leave scheme (with recall) at 60-70%-ish pay (no housing or ancillary allowances, etc.; preserving medical and peripheral benefits in countries which need it) there as well. That'd get you recoverable assets when things pick back up a bit.
A straight layoff is 6 months' pay (with in-seniority recall rights) in most contracts so it has to be something better than this (i.e. it'd have to be better than a simple one year at half pay in that 6 months is half a year so a person would be better off to force layoffs).
Give this a try and then bring down the axe.