from the instructions - With trench warfare you can only assault from regions held continuously since the start of your turn. If you conquer a region (or re-conquer a region) you cannot assault further from that region during the same turn. The one exception being 'killer neutrals' (explained on the Gameplay Notes page) from which you may continue assaulting.
So you should be able to bombard as long as you do it in the same turn as you take the killer neutral. See any Arms Race game with trench for examples of this.
08:58:39 ‹Razorvich› i have to say....its real FUNNY when you lot wake up for the day
09:22:32 ‹Swifte› **** that send her a lollipop and tell her to suck itJoin this doodle earth tournament!!!