Enable text funtions for custom expressions

I am having a filter that has a long name. 

E.g. 'Performance Yaw'. This is to distinguish for the users between 2 types of analysis, e.g. Performance or Reliability. 

Now I am trying to use it in a custom expression where I want to use always the substring(x, 13, 100), 
in this case Yaw. For e.g. a plot on the y-axis. 

It is not an existing feature, because I asked this to Spotfire Support with the answer: 

Thanks for contacting Spotfire support team.This is Manoj Singh from Spotfire support & I will assist you with this service request.

I have gone through your requirement but unfortunately, as of now, the custom expression does not get evaluated after 'as' or when used in aliasing. This case has been determined to be an enhancement request.