Ideally I'd like to do the latter, to build once then deploy to a number of locations with different root paths.

We discussed this today and we think it might be possible. Interestingly, Retype was first built with the requirement that all links were generated with relative paths, exactly as per your requirement to enable deploying the built site to any folder or sub-folder.

At some point we ran into a problem with supporting only relative pathing and had to revert to having the base config. As of v1.8.0, the base config was simplified into the url config and a separate base is no longer required.

We have to dig deeper and will run some experiments. I will keep this thread updated with our progress, although this is a relatively large change and is going to take some time to get working, but we do want to see this working.

Thanks for the feature request.

At some point we ran into a problem with supporting only relative pathing and had to revert to having the base config. As of v1.8.0, the base config was simplified into the url config and a separate base is no longer required.

Relative paths are a pretty fundamental basis for all forms of content systems and hierarchical systems in general. I'd urge you to revisit the decision to generate absolute paths across the board.

Is the retype build --override option applicable in this scenario?
I haven't experimented with it and haven't found any docs on the syntax for that command line option. Is it a whole file override or can you override individual config values?

Is the retype build --override option applicable in this scenario?

Yes, --override might be able to help. Individual config values can be overridden, including url.

We have yet to get to the feature of outputting relative paths. We will continue to keep this thread updated with our progress.