Just my 2 pennys worth. I maintain public rights of way accross Lincolnshire and Cambridgeshire. I drive down footpaths, bridleways, byways, farmers fields, the lot, pulling a fully loaded 14' twin axle ifor (weighing in at about 2500kg total (ok, not fully fully loaded)) behind a td5 disco. I've never really had any issues, even in severe mud. Ruts are manageable but a Sankey would handle these much better than mine. Ive only had to take the mower off once in 5 years to get up a hill.
Basically, I personally can't see how you'd have many issues with a 1000kg trailer, unless of course you go for some extreme offroading but is this actually likely during an expedition?
I guess one thing I would say is that you have to go a little faster in the mud to keep the momentum. This of course increases risk of damage.
One idea I had for a trailer would be to have two drop down wheels at the front so that the trailer could be winched independently if there was a situation where it had to be unhooked. Basically uprated jockey wheels with nice big tyres. Jamesmartins idea is a good one too. Some beefy hydraulic system with again updated jockey wheels.
