As a quick work-around, you can use 3dBrickStat to compute a sum. 3dBrickStat computes simple statistics over whole datasets, but individual sub-bricks can be specified as usual.
3dBrickStat -sum -mask mask+orig -dset+orig'[0]'
You could also multiply by some large number to make the means higher and then correct for it with ccalc which does allow you to specify the precision output format. You will probably also want to be careful if you are dealing with very small numbers in a dataset with other larger numbers to use floating point format for your data rather than short integers.