This is a good question!
First off, I should explain that we want to eventually add a second project variant which combines both metadata and the original photos in single location: i.e., if you import a photo, Tropy would make a full copy of the original and store it in a bundled folder, similar to how Apple Photos manages its library. This approach allows for more portable projects at the cost of flexibility (you can't spread out the photos across disks or even devices, cater to the requirements of other tools, like an existing Apple Photos library etc.).
That said, we are planning to add detection of missing files (e.g., because you moved an original) and the possibility to fix the link for each photo individually. Obviously, this is only practical if you move the odd file, but not if you move your entire library.
Therefore, we are also planning to store the last known location of the project file in the project's metadata: this way, if you move both the project file and your photos together, Tropy would be able to update the links automatically (as long as the relative paths between photos and project file stays the same).
Another possibility that we've been considering is to add a configurable 'photo repository base' path to each project: this way you could configure a base path independently of your project's location. If you then move all your photos you could just update the base path for Tropy to find all photos again.
Finally, we could expose a kind of filter function, based on regular expressions, to be applied to all file paths in a project.
I think that sums up our current thoughts on the subject.