Would you mind providing an example of the command you are using? If I create a 1 voxel mask doing the following, it seems to work with 3dROIstats, but I may not be understanding what it is you want to do.

# create a dummy dataset for testing
# tt.1D has one row with the line "50,50,50"
% 3dUndump -dimen 100 100 100 -prefix t -ijk tt.1D
# count the number of non-zero voxels in the dataset
% 3dBrickStat -count -non-zero t+orig.
1
# take the mean of the dataset using the same dataset as a mask
# it should have only 1 voxel with a value of 1.0
% 3dROIstats -mask t+orig t+orig
File Sub-brick Mean_1
t+orig 0[#0] 1.000000