I decided to measure out and store quantities for each wagon separately. I purchased some small cheap containers from Woolworths and they now each hold one hopper's worth of 'coal'. I also fabricated two funnels to assist the loading and unloading, the long rectangular one for loading and the square one for unloading. In reality, the unloading funnel could be any size as long as the coal can be poured from the hopper without spillage. However, the dimensions of the loading funnel were a little more critical. The base of the loading funnel is slightly smaller than the top of the hopper and two styrene supports were glued to the base of the funnel so that it would sit on the hopper.
It's a simple process that does not take too much longer than the loading of the other CCH loads but it is advisable to spread the load along the length of the hopper. To date, there has been very little spillage of 'coal' and any major issue will be addressed with 1:1 scale vacuum cleaner.
I was a bit concerned that, perhaps, I was under filling the wagons, but after a check of a loaded coal train passing through Hornsby, I noted that quite a few hoppers were significantly less than brimming. So I don't think it's an issue.
This idea may not work in hurly burly of an exhibition or on a large home layout during a major operating session. However, it is viable for layouts like Philip's Creek with a single operator. Would I use the same technique for the CCHs? Probably not, it may be more realistic but it's not really necessary and poor old 5131 struggles with heavier loads, so I think I'll restrict it to the five LCH wagons in the fleet. However, the concept may be trialled for the BBWs where I am experiencing a similar challenge.