I'm not a DAM expert ... just trying to manage my files.
If we don't want to manage the output image files in the DAM I think your proposal is great. In the DAM every output image is visible and if edited from there, photoflow will reuse the source image. Only a thumbnail may be a bit short. I would prefer a preview, to be able to have some evaluation of the image from the DAM.
In that case would we keep the pfi itself ? or only the pfi.jpg file ?
On the other hand if we want to manage the output image files (jpeg, tiff ..., this is useful to apply to them keywords, etc ...) we will see in the DAM twice the same image (the output itself plus the pfi.jpg file) everytime. I think that adds some complexity on the DAM size.
The xmp container alternative is probably cleaner. It could be applicable, not only to a thumbnail but to the output images themselves jpeg or tiff. Every output would include the reference to its source and the treatment which produced it.
The problem here is when we want to publish the output. We need to have the option not to include the pfi information or a tool to remove them.
That's where the pfi file, seen as output sidecar (I apologize if I embarass you again but I don't find a better word), may give more freedom while providing the same services. We see the images we have created in DAM. Opening it with Photoflow, we can reedit the treatment. We can publish the output without concern about the treatment data.
There is another topic related to this if we talk about DAM. It's the treatment of metadata and how they flow across the different tools. About this, does Photoflow transfer the metadata from source to output ?