Etihad have some stonking Business Class bargains exBRU for SYD right now:
£1275 BRU-SYD rtn all in; seeing as a stress free jaunt to BRU can be had for peanuts on the Eurostar, this seems worthwhile.
I have never flown with them, but the fully flat beds (soon to be re-invigorated) look great:
Not certain you get these fully flat seats exBRU, which could explain the bargain pricing.
How easy is it to get from the Eurostar Terminus to the Brussels Airport?