I have a bag of carbon in my sump and have had no ill effects. I keep mine in a higher flow area. I've also heard of people putting them in the skimmer on top of the bubble plate but have never tried it.
If you already have a reactor running Rowaphos then why not add an extra sponge and run the carbon in there are well? Unless the reactor is too small to run both.
The carbon does a much better job of removing crud from the system if the water flows through instead of over it.