I emailed and received a response from Pierre Bernard, the developer of HoudahSpot: “HoudahSpot relies on the Spotlight engine. This in turn relies on an index to find files by metadata or text content. In the absence of an index, HoudahSpot can often find files by their basic file properties like name and creation date.” So, apparently, Spotlight does not index an open disk image.
With that in mind, a post by @tjluoma on another thread got me to thinking:
I downloaded MacPilot (via Setapp) and was able to successfully enable spotlight indexing of specific open disk image files. Once I did this I was able to search and find files by tags and spotlight comments both in HoudahSpot and through Spotlight directly.
I believe that this stresses the importance of having a good file naming system even if you use tags or spotlight comments. You never know for sure that you’ll be able to search by tags or spotlight comments wherever you may store these files. Can you search by finder tags, spotlight comments, etc on OneDrive, Google Drive, or DropBox? I have not tested this.