If the wall does not have a break, I have seen a pillar installed to make the break. You can stain it or paint it, whatever matches your existing trim. If you can, take a digital picture and send it to me.
Sfritz@cinci.rr.com
Thanks
Scott

IDEAS 1) BUILD AN ARCH TO SEPERATE THE ROOOMS.
2) CUT A PIPE IN HALF, PLASTIC , OR A PAPER TUBE THEY USE TO FORM CEMENT COLUMNS. AND PUT THE ONE HALF AGAINST THE WALL TO SIMULATE A HALF COLUMN.
3)WOOD MOULDINGS.

Welcome to Plaster Talk Steve. Could you send me a picture of the dinning room?
soip@bellsouth.net
Doyle