No, there is no way you can force a single file not to be written to disk. Writes to a file are passed down to the filesystem code: if the filesystem is backed by memory (e.g.,
tmpfs) then data is written to a (virtual) memory location; if the filesystem is backed by a block device (e.g., ext3/4) then data is written to the block device.
You can, however, symlink a specific file to a location in a
tmpfs filesystem; this will work as long as: (1) the destination of the symlink exists, and (2) the link is not removed and the file re-created in the original (disk-based) filesystem.
~/.xession-errors, there is some special treatment in
/etc/X11/Xsession (lines 63--72 as of
x11-common version 1:7.5+5ubuntu1): if
~/.xsession-errors is already a symlink, then a temporary file (in directory
/tmp) is used instead. So, if your
$TMPDIR filesystem is mounted from RAM, you're done.