User Picker custom field should have default option of 'Current user'

With custom user picker fields, you can choose particular people as the default user.

But I would like to be able to have the default user be "the person executing the transition that caused this screen to appear". The default should only be applied if the field is empty upon the transition starting.

Currently I am achieving this using some ajax and javascript code, but it doesn't always work.

I have investigated transition post-functions but they're not ideal, because I want a user to see that they have been set as the value for the field, while also having the option to change it to another value. Using a post-function either over-writes their value, or even if it doesn't, it looks odd to the user to leave a field blank and then have it automatically populated with their name when they click save - better to auto-populate it with their name when the form first loads so there is no confusion about what's happening.