The 3-component/value thing is hard-coded as you say. However, you can pass the output through rmtxop to perform the conversion you are after:
dctimestep .. | rmtxop -c .265 .76 .065 - > result.mtx
If you want to incorporate the 179 conversion to photometric units, you would use:
dctimestep .. | rmtxop -c 47 120 12 - > result.mtx
If you produce binary values, either of the above would be faster as a result, but just as fast and using less memory is rcalc with getinfo:
dctimestep -of .. | getinfo -c rcalc -if3 -of -e '$1=$1*47+$2*120+$3*12' > result.mtx
This trick only works with binary matrix files, because there are no end-of-line characters to worry about. I’m also a bit worried that the NCOMP= line would be incorrect, which you could fix with getinfo and the -a option, but it would mean another command in the pipeline.