The referenced FoE "filter" is actually a "pre-filter" for items that will ONLY be visible to other members in the same Fellowship. The obvious use is favorable trading ratios.
FoE also has a back end filter for Guild posted, Neighbor posted, and adequate quantities. Note that there no filter for non-guild trades that would violate guild policies. As a closely related matter, preferential trades can be safely presented if the receiving party posts a maximum ratio trade in their favor, which the donor then picks up in accordance with a prior agreement. That works fine, technically, but socially it's not very effective because the donor can't just shotgun "favorable" feel-good trades, that nobody actually wants, while the recipients are typically reluctant to post such requests because they don't want to incur the consequent social obligations.
Therefore, I'm not at all persuaded that the filters would be a good idea. Our value ratios are capped at 4:1 through 1:4, uncovered trade are sorted last, and the idea of EXCLUDING the neighbors really bothers me. I'd much rather have vibrant neighborhoods, and I'm reluctant to sabotage the neighbors. I'm equally reluctant to encourage the Miss-Goody-Two-Shoes aspects of Fellowship Only trades, because of the pernicious effect that it has on the social dynamics within a Fellowship.