Hi Will,
The ability to sync attributes other than the ones provided out-of-the-box is not available in Clarity PPM 15.5.1. This was added in Clarity PPM 15.6 for custom attributes. The product release guide covers this under the following section:
Clarity PPM 15.6 Release Notes - CA PPM SaaS - 15.6 - CA Technologies Documentation
It lists the feature as available for Custom Investments, but it will actually apply to all investments, including Projects and Ideas (also a new feature).
The sync'd attributed do not need to be individually identified, rather, you only need to ensure that the REST API ID are consistent from Investment object to Roadmap Item object and that the data types are matched (for lookups, this implies the lookup also matches).
If you have a 15.6 environment available, please give it a shot and let us know how it goes.
Regards