[tjnycum@nx1 Data]$ AFNI_COMPRESSOR=GZIP 3dcalc -a EPI_All_Runs_mean+orig. -expr 0 -datum byte -prefix blank+orig
++ 3dcalc: AFNI version=AFNI_2009_12_31_1431 (Feb 25 2010) [64-bit]
++ Authored by: A cast of thousands
*+ WARNING: input 'a' is not used in the expression
*+ WARNING: output sub-brick 0 is all zeros!
++ Output dataset ./blank+orig.BRIK <--------
[tjnycum@nx1 Data]$ ls blank+orig.*
blank+orig.BRIK.gz blank+orig.HEAD
Easy work-around, but at first sight I was afraid it hadn't worked. Note that the Output dataset quoted in stdout is incorrect.