I have done this before with good results. If I reconstitute the powder to make "egg whites" I strain it before using. Just using the powder in the same way as making royal icing is easier and works great.
HTH
Michele
I'll second that. I prefer liquid egg whites --they always whip up for me, except for All Whites (i.e. the name brand) does not. I did a taste test once: real ew, liquid ew, and powdered. There was a difference (with real ew being the winner, liquid in 2nd place) but it was almost imperceptible, and I don't have an issue using powdered when that's all I have on hand.
Thanks for the tip on straining though, Michele...occasionally that is a problem for me and it never even occurred to me to strain it! 
Quote by @%username% on %date%
%body%