I think you may create a Text box with "All" as text and show it on top of the multibox's field selection area. This text object shd be visible as long as getSelectedCount(Country)=0.
Another way to have the value of field Country as All will require a load script change you can create an inline load like below:
load * inline [
Then in your qvw you can create the multi box with adding the Country field. have the sheet to be triggered to the field value flg=1. That means when sombody comes to the sheet then the flg shd be set to 1. In that case the indirect selection of Country will become "All" in white color.(Possible value)
Can I ask why you want to do that? QlikView does not have a steep learning curve, so as soon as users understand the concept of green/grey/white they are ready to go.
With your approach you risk setting the wrong user expectations. Every time you develop a dashboard users will expect "ALL" to appear in their lisboxes and multiboxes, which means more development and complexity for you.
Just playing devil's advocate
Kai is absolutely correct. There is always a way to manipulate the UI or data model to achieve what Client has requested but I think the first priority should be End users training on how this tool works. As Kai mentioned it is very simple and st forward to learn the tool from an end users point of view. I think you may think of educating the end users on the tool rather than changing something in the behavior or appearance in the UI.
Kai thanks a lot for bringing the concern to our attention.