Assuming you have real valued voxels in ${seed} that you want to use as weights in addition to their defining the mask, something like the following should do what you want.

3dcalc -datum float -a ${seed} -b input.nii.gz -expr "a*b" -prefix output
3dmaskave -quiet -mask ${seed} output+tlrc.HEAD > ${seedName}.ts.1D

However, a very useful feature to 3dmaskave would be the addition of, for example, a -weights arguments that made 3dmaskave multiply in the value of each voxel in the mask against the corresponding voxels in the input dataset prior to the computation of the summary statistic would be fantastically useful.