The problem with rhubarb is not only to get the juice out, but it is also the high acidity.
Getting the juice out is the least of the problems:
Freeze it for a few days and then thaw it.
The rhubarb will be so soft that you can put it in a cheesecloth and press it by hand.
Lowering acidity is a bit more complicated.
You can dillute the juice but then you will be dilluting flavor also and I
am not an advocate of that.
You could use chalk, but chalk will give an off taste when used in great amounts.
So I have devellopped another method which idea was initiated by a Belgium winemaker called Gocar.
You freeze the juice again.
Now when frozen you will see dark spots on the icelump.
That is were the acid is concentrated.
Cut them away with a knife and you will have a pure juice low acid rhubarbjuice.
The method I describe here can be found on my web-log (you really should read it all more often) in this entry:
http://wijnmaker.blogspot.com/2007/06/scroll-down-for-english-text-al-lange.html
And I made 2 different wines using this method which both came out great. For both I used a slightly different procedure you can read them here:
http://wijnmaker.blogspot.com/2009/09/rabarberwijn-1-rhubarbwine-1.html
and here:
http://wijnmaker.blogspot.com/2009/09/rabarberwijn-2-rhubarbwine-2.html
As usual the method and procedures are clearly documented with photo's.
Luc