I've been working with the late Mennonite theologian John Howard Yoder's book The Royal Priesthood: Essays Ecclesiological and Ecumenical in connection with preparation for an upcoming paper presentation. Among several gems I've encountered therein are a couple of passages in which Yoder points to the unrealized potential of the "gathered church" ecclesiologies of the believers' church tradition for contributing to the quest for visible Christian unity:
"This view gives more, not less, weight to ecumenical gatherings. The 'high' views of ordered churchdom can legitimate the worship of a General Assembly or a study conference only by stretching the rules, for its rules do not foresee ad hoc 'churches'; only thoroughgoing congregationalism fulfills its hopes and definities whenever and wherever it sees 'church' happen" (p. 236).
"The locus of visibility of most Christians is where they live and go to church. Therefore, the most important locus of concern for unity to be visible should be on the home level, in the relationship between Christians across the back fence, or in the same school district, or between neighboring congregations of different confessions" (pp. 297-98).
Inasmuch as a major contributor to the current ecumenical impasse has been the focus on dialogue between official representatives of world Christian communions (which tends to privilege churches of non-congregational ecclesiologies) to the neglect of local grassroots ecumenical engagement, Yoder may have been on to a way in which the churches of the free church tradition can make substantial contributions to the modern ecumenical movement through their unapologetic involvement in it.