I finally got around to tweaking the implementation of nrrdSlice (called by unu slice) and nrrdProject (called by unu project) so that you can work on 1-D arrays. The results in these cases are single-sample 1-D arrays, which is unusual, but more useful than generating an error in these cases. Adding the special case of slicing/projecting a 1-D back to another 1-D array was a far less disruptive change than, say, allowing nrrds to have dimension 0. Do an svn update on a Teem trunk checkout to see this change.
Examples with attached image:
# find value[0,200,120]
unu slice -i skull.png -a 0 -p 0 | unu slice -a 0 -p 200 | unu slice -a 0 -p 120 | unu save -f text
# find average value of whole image
unu axmerge -i skull.png -a 0 1 | unu project -a 0 -m mean -t float | unu save -f text
Prior to this update, you'd have to "unu axinsert -a 1 | " prior to the final slice or project, which got to be annoying after about a decade.
Behavior on higher-dimensional arrays should be completely unchanged. Try it out and let me know if it works as expected.