redirecting /tmp

[Robey Holderith]

I'm trying to find a way to set an environmental variable so that the 
system will use /usr/tmp or something instead of /tmp as a temporary 
directory.

The story is that I was attempting to change the size of /usr remotely. 
  I backed up all the data and then copied the bare necessities over to 
/tmp then changed fstab so the drive formerly known as /usr was never 
mounted and /tmp was mounted as /usr.  Great! it worked... but now su 
isn't working... because now /tmp is 755.

Any insights are welcome.

-Robey Holderith