Y
Don't quote me on this but I believe the issue with that is that, if you could make creatures which have no combat AI, have correct faction settings, and look like barricades, is they wouldn't effectively block movement in the same way that placeables do.
Placeables, on the other hand, can block movement. But faction settings can't be placed on them such that the AI will attack them on sight.
I think that the best middle ground, easiest to implement option would be Dermin's suggestion: barricades which are only usable in certain, frequently sieged areas, such as Norwick's gates, where hostile spawns do not appear without DM oversight to make them react appropriately.