Hi Richard
Interesting question

After reading you post with intrigue, my first thoughts were that perhaps a thinned down tomato soup could be used as an emergency curry base .....
..... then I thought, where are the onions? Maybe onion soup would be better ....
..... what about the vegetables. Ah I know, we use a vegetable soup ....
..... and the spices, garlic, ginger, oil ... well ........ we might as well make our own soup and we'll call it a curry gravy base.

So, in answer to your question Richard, I think maybe if you combine a tomato soup with an onion soup and a vegetable soup, add lots of water and oil, some essential spices (and maybe some ginger and garlic), boil it for a while, then liquidise, this may be something reminiscent of a reasonable curry base.
Worth trying ... nah. Costs too much and far too complicated.

Regards
SnS ;D