Where can I publish the proceedings articles of a small conference?
We ended up publishing the proceedings through ResearchGate, using this procedure:


The authors send their submissions to the proceedings editor for approval.
The authors upload their approved submissions to ResearchGate, following a guide provided by the editor to ensure that the metainformation is consistently registered.
The editor uploads a proceedings index to ResearchGate, which gives an overview of and links to the proceedings submissions.


This ResearchGate procedure has some advantages:


Fairly straightforward, both for editor and authors
Good searchability: The proceedings submissions are automatically indexed by Google Scholar
Good visibility: ResearchGate helps by suggesting the proceedings submissions to other relevant researchers


It also has some disadvantages:


Unlike the arXiv procedure, the individual proceedings submissions do not link back to the index.
The authors are required to register ResearchGate account and do the work of uploading their own submissions. (Unlike arXiv, ResearchGate never allows uploading by third parties such as a proceedings editor.)


All in all, this approach was quite successful and we are fairly happy with it. I imagine we will use it again next year. Of course, however, this is not the only possible approach, and I would still be interested in other suggestions!