I need to pipe some words on my MMF "Welcome Atticus" to be exact. Can I do this with my buttercream? I have to drive 45 minutes with it once done, no time once at location to do it then. Have never made RI so I don't know how. BUT, I want to do it the best way. It needs to be blue too.

Yes, you can use buttercream.

(you didn't mention if the piping is on the top or the sides, so if it's on the sideyou might need to prop your cake up a bit while writing at eye level.)

It's on the side but I think I can do it before I attach the piece to the cake.