Hi Mark,
the _SYS_BI schema, like all _SYS_* schemas are special system schemas and users are not supposed to change the definition or the contents of the objects in these schemas on their own.
With the next revision upgrade any new or changed column is likely to be overwritten again, anyhow.
Therefore you might be best off to stick with the much more flexible approach that you are using currently.
Also, you should think about formatting the actual output value in your report front end instead of doing it in the model. Ultimately you don't want to set the output format of data in the data model but keep this decision to the report design level.
- Lars