That would seem like a reasonable option to have there, but those are currently the only options: Extra Axis: Labels
Here are some suggestions for now …
If you are replacing the standard axis with an Extra axis, then you can still use the labels in the main Axis settings, where you have the default ‘y numbers’ not drawn on the axis.
Here is an example where the extra axis is replacing the default to convert the units on the axis tick mark labels.
Here is another example with the Extra axis on the right where you would want to have the label in the center. The only way to do this currently, is to add a Text command to add the label, with the following settings.
With so many formatting options in the Text comment, hopefully you can get the label exactly where you want it to go, but it does seem like a lot of steps. We will think about adding other options in the Extra Axis label.