It's true that the AHDL's are starting to be cut up - they are such poorly designed vehicles anyway that their commercial value is probably nothing and they leak all over the place.
However GWA does own a fleet of BG-capable hoppers - the AHGX's which have been on lease to the Sword of El-Zorro for some time now.
The CGAY hoppers from CFCLA are probably dead-easy to convert to BG, but there needs to be a viable business case to do it. Consecutive good grain harvests would make that possible but that's something that Australia hasn't had for quite a while. And there needs to be more than just grain to keep the lines sustainably open for traffic. Tourist trains are the way to go there especially up my way.
No idea what's going on with them, and I think most of them are on standard gauge anyway since El-Zorro has run quite a few grains trains to Mallala in recent months.
What's important is that the track is still in place. Whilever that's the case, the ability to re-use the track for trains is much more feasible than if the rails are removed. The destruction by and ignorange of ARTC relating to the removal of the broad gauge connecting Belair to Mt Barker Junction when the line was changed to SG is a notable example of deliberate prevention of unwanted lines becoming commercially viable again.
Be careful that now the world economy seems to be recovering, the price of scrap steel will rise again, and that means closed lines could be ripped up to re-use rail or sell it for scrap value.
That's the big danger at the moment. SG conversion of the Gawler line is definitely in the wind, and that would isolate the BG lines, but whilever the BG is still there it's potentially available for use for grain trains and any other freight and passenger traffic which was viable.
The Canadian company that bought ABB is very pro-rail apparently, so there could be some weight in favour of at least giving serious thought to re-vitalising some of the remaining BG lines.