Thanks for the info everybody, getting involved with this one does sound like it could be a bit of a lottery.
Unfortunately the design incorporates all of the elements that will work against it, i.e lots of crevices to catch and build up detritus, an inability to get inside and apply any sort of coating once finished (without dipping that is) and potentially being installed in an estate in the UK.
Perhaps I could push for an industrial galvanised finish instead, although dipping something the size of a heavy horse would be something in itself.