Keep an Experimental draft that documents how we can add pii attributes to different log formats. Logs are inevitably heavily customized anyway, so having an Informational/Experimental document that aims to advise on how one can modify one's log formats to fulfill X use cases would be a natural fit.
Publish a Standards track document that adds a new Structured Data ID value for our use: https://www.iana.org/assignments/syslog-parameters/syslog-parameters.xhtml. Look at syslog Structured Data ID Values and examples of RFCs that add SD-IDs. We can have a very small, tight document if we do this, but will have to show a lot more interest from people willing to implement.