I reckon what you needs are two vessels that fit inside each other. Drill out a bunch of holes in the bottom of the inside one and line it with voile.
Drill 4 holes in the inside vessel's perimeter and install chains.
When your mash is done hoist the inner vessel out, put a plate of plywood of the same diameter in and a few kgs of weight, let it drain, swing it out of the kettle's way and unclip three of the chains letting the grain tip out into a wheel barrow.
I reckon this is the way to go so that sparging is possible. I see three options.
1) Use a 140L pot for the kettle, buy a 120L pot to fit in that and drill holes in the bottom. Would probably work really well, but it would be expensive, plus the 120L pot would never be able to be used for anything else.
2) Use a plastic inner vessel. Cut the top off a plastic 44 gallon drum, drill holes in the bottom and use that for the inside vessel. This means that you can sparge correctly by tipping in water. Obviously the material that the drum made from would have to be food grade.
3) Use a bag that has impermeable sides. A bag like this would be exactly the same as using a bag in a pot with holes in the bottom. The material that i think would work would be hdpe, like those woolworths green bags. Those bags cost around 10c to make so it shouldn't be too expensive to make one up. Making a bag with woven hdpe sides has the advantage that it can be exactly tailored to the kettle, as it can be made as large or small as you want. The sides will also add strength to the bag. So you could just lift the bag up and throw your sparge water on top.
Also, for lifting, i reckon the best thing would be a cheap hand winch, like this one.
http://cgi.ebay.com.au/2000LB-RATCHET-HAND...=item2a0d3fff75
And also a pulley system. A quad batch of wet grain is going to be bloody heavy @ 30-40kgs!!
I know someone up here who has bought a 140L pot so i will check their's out. If i can find a source of that 'green bag' material for the right price then i think i will seriously consider making a massive biab rig. Even if i don't upgrade i may look at making a bag with impermeable sides for my double batch system. I think it would make sparging so much easier if i could just hopist the bag up a little and pour the watter in the top instead of f###cking around with plastic containers and pots of water.
The other thing i'm going to try next brew day is using two bags in my double batch system, just to see how it goes. I have a feeling its going to be a little frustrating, but it might be really good so we'll see.
thanks everyone for their input, there's some really good ideas here.