Alan, there was a discussion about this a while ago on the Colonial Wars Yahoo Group (I think you are a member there?).
My contribution was remembering a YouTube clip of Gunga Din which had guns being carried on the backs of elephants. This was followed later in the film by Gatlings being brought into action off the backs of elephants. There was some discussion on the group about whether real equipment or props were being uses, if the harness would be sufficient to bear the weight etc. I have just checked and unfortunately this clip is no longer available (there may be others)
I also put some scanned images of an article from Soldier magazine by Bill Connolly October 1969 into the photo albums section (under20mm Colonial Collections) which had some pictures of elephant drawn and carried artillery – the focus was on gatlings but it might be of some interest. They are in the photos section.
I haven't seen it but apparently there is a book called War Elephants by J M Kistler which has an engravingof guns being loaded on elephants.
In the end of the discussion the main conclusions were:
"The 1850's loads seem to be:-
barrel on the first elephant,
trail on the back of the second elephant with the wheels detached and hanging on the elephant's side.
There are other elephants in the background that could be carrying disassembled limber and ammunition.
I can't say for certain what the piece being carried was. It is captioned as a small cannon but a brass 6pdr barrel would weigh 6cwt compared with a 9pdr at 13.5cwt.
The 1897 loads for the 12 or 15 pdr. RBL are:-
limber with attached wheels but with the trail pole disassembled,
barrel on the second elephant,
trail on the third elephant with wheels detached and hanging in similar manner to the previous example.
The 12pdr RBL barrel started out weighing 7cwt but a lighter barrel of 6cwt was developed for the Horse Artillery. The weight of shell was
increased to 15lbs for the field batteries.
Your suggestion of 1000lb per elephant holds good for a 6lb in 1857 and either a 12pdr 6cwt or 15pdr 7cwt in 1897.
Other maximum loads for pack animals in India were 120lb for an ass,160lb mule or bullock and 400lb for a camel."
You can find this message, in the context of its original thread, by searching for Message #87284 on the Colonial Wars Group
Hope this helps