Starting with R2023a this is possible by using the Graphical Icon Editor. In the attachments of this article you can find the Simulink model files which are created with this workflow.
Here you can find the documentation of the Graphical Icon Editor:
In the following a short tutorial can be seen which describes the general process:
1) Creating a Mask
At first create a Mask on the block which should get this in-place parameter editing feature.
At first right click the block. Then go to "Mask" and then "Create Mask..."
2) Adding a Parameter to the Mask
Because you want to change a parameter in the block, you would need to define one.
Add one Parameter widget from the left to the "Parameters & Dialog" area.
In this example a Edit widget is used. Since we are using a MATLAB Function block which should output the parameter "u", it is needed to change the "Name" from "Parmeter1" to "u".
3) Open the Graphical Icon Editor
Now open the Icon Editor by clicking on the Icon tab and then click on "Drawing Commands" to change the "Authoring Mode" to "Graphical".
4) Adding "Text" to block mask
Now you should see the Icon Editor. Add a "Text" Element to the block mask at the position where you want to show the parameter.
5) Opening "Text Paramterization" configuration
Now in order to configure this "Text" element to be editable on the canvas, right click this element and select "Parameterize".
6) Changing "Dynamic Text" configuration
In the bottom right you will now find a new configuration pane. In this add the name of the previously added parameter "u".
7) Changing "Editing behavior"
The last steps are now to select "Editing behavior". As "Edit Handler" select "Parameter".
Also here enter "u" as "Parameter Name". Now press "Save Mask" in the top left to save the configuration for this block mask into a svg-file. This file is needed to show the block icon.
5) Ready
You can now close the Mask Editor/Icon Editor and verify if it works.
