Could be worth a try as well Feldon. Tartaric acid / Cream of Tartare comes up quite frequently, as does citric acid and of course lemon juice. The sultanas also feature in a few and I dropped a few in my first attempt at creating a Ginger Beer Plant.
I was reading through some of the forum's older GB threads, and I did find mention of others using the jarred 1 kg "Indian" ginger paste. I can't find any discussion about the end results though. I'd be very interested if anybody has used it successfully. The jar I have is from Katoomba foods and it contains Ginger, sunflower oil, salt, vinegar (water added). The oil content in 3.5g and there's 835 mg sodium / 100g. I don't see too much of a problem with the salt, none at all with the vinegar, but the oil worries me. Although, if it rises to the surface in fermentation and you rack the beer off from underneath it may effectively remove most of it.
I guess I'll just start another experimental 1.5 litre brew to see what it gives. $6.60 / kg is a lot different to $25 - $35 for fresh.
Edit: If you like the commercial soda tasting GB and haven't yet sampled it, try to get your hands on a few cans of the BROOKVALE UNION GINGER BEER which is, according to the can, Produced across Australia by 4 Pines Brewing Co. One of my son's mates left a 6 pack in an esky after NYE party and they are delicious. What I'm hoping to be able to replicate, or get close to anyway. Mind you, these are a manufactured drink, not brewed from what I can tell of the ingredient list. 4% and 1 Std drink in a 330 ml can.