Allow non-numeric measure numbers

In some cases, especially with musicals, as new versions are created and parts are adjusted, there may be a need to add measures before prior numbering begins or in-between numbered measures, and the original numbering should be preserved to keep all parts (including those already printed) relatively in sync. In these cases, it would be useful to be able to set the measure numbers to an arbitrary string, such as 'A' or '72b'. (See attached examples)

This would be an option on the individual measure itself.