What is the recommended way to store documents in proprietary formats (such as .mindnode files from MindNode 5) in DEVONthink?
So far I’ve tried the following:
Approach #1: Import .mindnode files into DEVONthink.
This works really well in DTPO but in DTTG viewing or editing the files is cumbersome. Viewing in DTTG requires “Copy to MindNode”, which creates a copy, while editing requires opening the MindNode application and manually navigating to the file using the document browser.
Approach #2: Keep .mindnode files in iCloud Drive, index them in DEVONthink, and set URLs for indexes to "mindnode://open?name=ThisMindNode…” (so that the mind nodes can be accessed in DTTG via the URL scheme).
The problem here is that indexing files in iCloud Drive seems like a bad idea (since on another Mac the file paths will be different).
Approach #3: Keep .mindnode files in Dropbox, index them in DEVONthink, and set URLs for indexes to "mindnode://open?name=ThisMindNode…”.
The problem here is that the MindNode URL scheme seems to only work with files in iCloud Drive and can’t be used to open mind nodes stored in Dropbox.
Approach #4: Keep .mindnode files in iCloud Drive and create bookmarks in DEVONthink with URLs set to "mindmode://open?name=ThisMindNode…”.
This has the drawback of not providing Quick Look previews in DTPO, although I suppose some kind of triggered script could provide those.
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Some of the above is specific to MindNode 5 but I am also interested in importing OmniGraffle and OmniOutliner files.
Addendum/correction:
It seems that the directory names in ~/Library/Mobile Documents/ are actually stable and will not change from one Mac to another. For example, iCloud Drive MindNode documents are always stored in:
~/Library/Mobile Documents/W6L39UYL6Z~com~mindnode~MindNode/Documents/*.mindnode
I thought that the “W6L39UYL6Z” part would change from Mac to Mac, but this doesn’t appear to be the case.
The same applies to all of the other directories in ~/Library/Mobile Documents/.
In this case, keeping the .mindnode files in iCloud Drive and indexing them in DEVONthink seems like a pretty good solution. In DTPO the behavior will be completely seamless, with Quick Look previews inside DEVONthink and shift-command-O to edit in MindNode. In DTTG, setting a URL of “mindnode://open?name=xxx” for the index file will allow the mind map to be accessed by selecting the entry in DTTG, then tapping the info button on the blank “This document cannot be displayed” page, then tapping the Safari icon next to URL field.
I guess I answered my own question - the best approach is to keep the original files wherever the original application handles them best (iCloud Drive in the case of MindNode) and index them in DEVONthink. If I am still missing something please let me know.
The approach described above seems to work quite well, so now I have a feature request. 
When DTTG does not know how to display some document, and that document has an associated URL, then DTTG should open that URL instead of showing a blank “Document - This document cannot be displayed” screen.
Users could then fill those URL fields with custom URL schemes, so this functionality would be analogous to triggered scripts in DTPO.