Just post a reverse trades for what's needed in your guild. Doesnt usually take more than a few minutes if your guild has a trades thread. Would take less time than you'll spend trying to find out why they arent classed as normal goods for trades.
The special goods dont count as productions for gathering quests, although they are now available to produce in your city I doubt the devs would spend time programming in the change for one building. Its easier to just play in the knowledge that special goods don't count.