A couple of updates:

Update: @machniak informed me that it shall be simpler for Chwala to just 'register' the uuid to Manticore. Then, Manticore can send a request to retrieve that file when needed.

An update: Chwala may or may not store user login credentials for external storage based one what the administrator chooses. However, it becomes impractical to present a login dialog to Manticore users when they want to save a document.

So in Manticore's upcoming Chwala storage adapter,

1. CSS Regions - these are not really usable right now until all browsers roll it out. But we could try using polyfills to see how performant this approach is.
2. SVG <use> tags. Wrapping the entire document in an svg tag and rendering pages that aren't being worked on with <use> (a way to visually "reference" parts of an SVG without duplicating it), while displaying the "real" cropped SVG to show the current page. Not sure how doable this is, but it will let us do more tricks like arranging pages in different layouts (thumbnail bar for example)
3. Parse ODF page breaking attributes and just add "spacer" elements within document content to make it look paginated. Does not let us do the above mentioned tricks but is probably "good enough" and a safe approach. When CSS regions are ready we can probably switch to those without too much work.

1. Setting the exported snapshot buffer to undefined should encourage the GC to remove it from memory later. A noticeable improvement occured.
2. A genuinely significant improvement would be to exchange the snapshots over socketio instead of the node-phantomjs library's bridge (which is seriously inefficient and a minefield of memory leaks).

But isn't preventing them from saving back fine?

IMO this is Manticore's business, and we should avoid duplicating it's features within Roundcube files.

Just my two cents: Is providing a separate document list important? Why not simply use Manticore to view this list?

A further question arises, on whether a document to which I have read-only access should be annotated with the fact a collaboration session is ongoing on that particular document.

Robert opens the file (doubleclick) to join Susy in the session.

I agree, we should simply use the regular collaborative setup and not worry about providing a single-user mode.

[1] The most convenient solution would be to have administrative rights available handy to work on a shared folder. If, however, only a system that exposes individual rights is possible, then enough exposure of who has what permissions on a source document is required.

If "editing files continues to work" means live sessions will carry on fine, this is not possible if things like etags and last modified timestamps will change. Unfortunately moving to a different file backend will make us lose information that identifies files in the new "filesystem" with their versions in the older one.

Unscientific observations so far:

I haven't come across WebDAV servers that support ACLs yet, so the access control in Manticore so far is the best I could do.