If you're not starting with a Qatar Airways flight, then you can always try using the online booking tool
Good luck with that, though I did manage to use it back in March - see this post

The tool claims it can issue Global Explorer in addition to Oneworld Explorer tickets, but nobody has yet reported trying that

I expect January is the peak of the peak for people wanting to fly LHR-MLE, so BA might well restrict availability of discount business seats (which D booking class is)

If you want an out of the way tropical island, on a Global Explorer ticket Christmas Island is possible (but only one flight per week, so you'd be stuck there for a minimum of seven days)
(the Global Explorer ticket allows additional airlines, including Fiji Airways)

ETA:
re your SYD-SCL flight, which implies visiting South America
Are you aware that you can no longer include LATAM flights on a Global Explorer ticket? See this post