Swollen abdomen filled with fluid.. I am assuming the vet is sending out some of the fluid??? What is the horses bp? Did they ultra sound the abdomen? Tell the vet he can go on the veterinary information network for ideas from other vets... I wish I had more specifics I would talk to the vet I work with...
My guess would be congestive heart failure. Are the legs stocked up. Is the fluid in the belly inside the abdomen or is it ventral edema outside of the belly muscles? If it is ventral edema [that pits upon pressure with a finger], then I am betting on congestive heart failure. If it is, then there is not much you can do. We have put a couple of older horses down when that was the case.
Vet won't come out until next wed? What kind of vets do you have out there? If I call with an emergency my vet comes out that same day. It may be a couple hours later if he is on another critical call or in the middle of surgery but he gets out there. He's even come out late at night.
I called my vet at 9pm on a thursday evening and he was here 30 mins later (not sure how as it is a 40 min drive)
I've called him out for colics on a saturday night at 11pm and I called him out when stan came in lame on a friday night at 7pm. On the Sat night he was with me within an hour, on the friday night he was a couple of hours as he was out on a colic call.