It seems to me that an under-emphasis on the person of the Spirit is not simply a universalist problem. Rather, especially as of late, it’s a (perhaps correct) borderline cliché to claim that so-and-so does not give significant attention to he Spirit. But, the Spirit’s precise role has always been somewhat more ambiguous than that of the Father and Son, and I am not particularly surprised when any theologian does not devote reams of paper to explicating a robust pneumatology.
As far as the contention that Ballou and the 19th century American Universalists more readily accepted unitarianism because they did not have a sufficiently robust understanding of the Spirit, I think it is far more likely that the Ballou tradition adopted unitarianism out of its particular theological method. Ann Lee Bressler, in her excellent The Universalist Movement in America, 1770-1880, contends that Ballou et. al. attempted to walk a fine line between a commitment to the inspiration of the Biblical text itself and “Enlightenment rationalism” (supposedly in contrast to Channing’s Unitarians, who - despite holding many similar theological beliefs as the Universalists - tended to overemphasize the latter).
Additionally, no doubt in part because he was holding to a clear minority position (and one that many more conservative-minded Christians considered heretical), Ballou was not prone to place much weight on the authority of traditional creedal statements, etc. The result, I think, was that Ballou and friends felt free to look at the (esp. New Testament) Biblical text itself and recognize that trinitarianism was by no means obviously taught, and, indeed, that it appeared at times to imply unitarianism (and they were, of course, not the only ones coming to this conclusion at the time, and indeed many had come to it before them). Unitarianism also had the added bonus of being more easily squared with “reason” than Trinitarianism.
It was this combination, I think, that provides a far more likely explanation for the American Universalists’ tendency to unitarianism than an insufficient pneumatology (even though the latter might be the case as well). If there is a relevant theological critique to be made here, I think it would not so much lie in the realm of pneumatology as in ecclesiology and theological method (i.e. an under-emphasis on the authority of the ecumenical creeds).
Are you claiming that a Christocentric universalist would seemingly be required to affirm that the Spirit (at Pentecost?) is presently on/in all of humanity? If so, I must confess that I do not see why this would necessarily be the case. And even if a universalist wanted to affirm that the Spirit indeed was present everywhere (the Spirit blows where it will and all that), this would not necessarily entail her adoption of a pneumatology offensive to (certain) non-universalist orthodox sympathies, as she could adopt a qualitative distinction between the special presence of the Spirit in the present believer and the “working” presence of the Spirit in the believer-to-be. That is, it seems to me that the universalist could absolutely affirm that at Pentecost the Spirit came upon the believers and remains specially present in members of the universal Church of Christ, while nevertheless contending that all will eventually be included in this Church. There, the Spirit provides charismatic gifts and all the usual stuff of traditional Christian theology. This view would likely be more agreeable to a purgatorial universalism, although an ultra-universalistic version is likely possible as well.
I’m not even sure that Barth and von Balthasar could not be read in a similar way (or at least easily modified to say something similar). I can imagine them making a distinction between the salvation that Christ has objectively accomplished on the cross and (esp. in von Balthasar) in his descent into hell and what the Spirit is subjectively doing in-time in the members of the visible Church. On this view, it is not surprising that the Spirit manifested itself in particular individuals, for these are the members of Christ’s phenomenal body on earth! But this does not entail that those without this special presence of the Spirit are not, from an “eternal” perspective, members of Christ’s body as well, but merely that they have not yet been united to Christ from an earthly, presently-unfolding perspective. Maybe this works theologically and maybe it doesn’t, but it does not seem to succumb to the brief critique you sketched.
This will likely be one of the more exciting aspects of your work to me, as I think you’re right that non-Anglophone universalists (besides Schleiermacher) have been given far too little attention. Likewise, I agree that universalists should not - in the case that your arguments in favor of condemning certain historical universalists with theologically unacceptable forms of esotericism or Gnosticism are indeed sound and persuasive - balk at your findings. For, again, such an argument is not an argument against universalism itself, but at most certain forms of it.
Hopefully you are not forced to cut too much. I enjoy a chunky book!