That would be the most realistic solution; imo all Anchors and (at least 1) door to the lobby should be exempt.
Or just massively up the amount of stuff you can put in the large guild hall. That would probably be much easier than coding anchors to not count. We had to remove several nicely designed rooms to make space for anchors. Kick the grand guild hall count up to 6000 would be nice or even higher.
I think making anchors exempt might be easier. I'm afraid they'd break something trying to increase the item count.