Shortening a lath and plaster wall.

I need to shorten a lath and plaster wall. As you might guess where I need to make the cut is not where there is an upright! One side of the wall is in the bedroom so is accessible whereas the other is in the airing cupboard and I would dearly love to avoid taking this to pieces to add plasterboard and to re-plaster. The airing cupboard side is however supported by a wall at right angles to it - so maybe I have a chance.
I wondered if I could make access holes to the void between the sides of the wall and squirt in expanding foam (not the high expansion stuff which would probably balloon the walls) and this might bind it together well enough for me to make the cut and add an upright to support it?
I'd appreciate advice from those with more experience in these matters.
Thanks in anticipation.

 

Hmm. Well no help from the experts then but I did fill the void with expanding foam (after spraying some steam in to dampen the inside of the walls for better adhesion). I then cut through the wall with a jig saw using a long sharp blade -possibly a Bosch T101B. It all went very well. The wall held together and it wasn't all that messy - I connected the jig saw up to a vacuum cleaner. I had to resort to a reciprocating saw to cut through the noggins. Maybe it would have been the same without the foam but I'm happy with the process and the outcome.