Provide username function in Power Query - M or DAX calculated columns

Being restricted to just using username () in measures (or AllowedRowsExpression) means that you cannot use the context of the current user in slicers or page level or report level filters.

Just to be clear, the desired result is not data security (that can be done through roles), it is to provide relevance. We want to be able to show a page that shows (for example) My Teams timesheet hours, perhaps next to a report page that shows an overview of the entire company.

You can provide some neat filtering using measures containing username () , but in particular, you can’t provide slicers (or page filters) that are cut down to just the list of relevant items based on the user logged in.

It would be extremely helpful if you could access the username value either via a function in PowerQuery M, or to allow username () to be referenced in a calculated column using DAX.
Trying to use username () in a calculated column currently gives the error:

"CUSTOMDATA and USERNAME functions are not supported in calculated columns. These functions may only be used in Measures or in the AllowedRowsExpression."

Perhaps there are security, functional or performance reasons why this isn't possible, but thought it worth putting forward.