I can. You put the flag down then the pillar. Vwalah it's inside. I meant the question more as: Is it against the rules to hide your flag in a pillar?
far as i know, even the mods know bases with claim flags in walls and pillars, considering that its the safest way to hide them and that they havent blown up my base for doing that nor a slap on the wrist hiding it in trees is illegal as they've cleared bases in pve8 because of that, also if you know the way of placing them in permanent structures as well (which was patched out before i got to try it in singleplayer i crie)
I just question it because i read a ban appeal where the person got in trouble for having their claim flag in a pillar instead of in plain view.