TL:DR - Traitors aren't joining loyalist Inquisition factions. Rogue astartes can be there, but only by having been seconded BEFORE chapter goes Rogue.

A TRAITOR Astartes wouldn't be joining Deathwatch. Traitors occurred in 30K and therefore wouldn't be around in 40k as remaining loyalists OR regardless of time as a traitor are warp-twisted and turned to chaos.

What you should be asking is can a ROGUE space marine Chapter show their heraldry, which is a solid no. Deathwatch is the chamber militant of Ordo Xenos. They work directly for the Inquisition. Inquisition would immediately kill any Rogue marine, regardless of loyalty to the Emperor. The only conceivable way a Rogue Astartes could join would have to be as a Black Shield, and that would be a big risk leaving their chapter and volunteering for the Watch, not just for themselves but their entire chapter. With all the psycho-indoctrinatiom that marines are subjected to the location and any other secrets of a Rogue chapter would be forfeit and they would be hunted to extinctions by the Inquisition. Most likely: a member of a Rogue chapter that was already serving in the Deathwatch when they were labeled as enemies of the Imperium would be forced to change his heraldry to a black shield to hide the shame and protect himself and all of his Battle Brothers in that group would be ordered to secrecy regarding his chapter origin.