User friendly data selection

How about letting the user select folders directly inside the data browser and just have two buttons 'Load selection as background' and 'Load selection as sample', then grab the (up to) first 5 selections as string using GetBrowserSelection()? You could even create your own modal data browser for this very purpose with CreateBroswer/M, where you, for example, hide all waves, so the user has to select a folder. Read the help for CreateBrowser which contains example code you can start with. I hope that helps.

I have some pretty well developed code for filtering listbox contents based on text matching. I don't know if that helps for your application, but I am happy with how it works. See it in action here.

May get some extra mileage by combining text filtering with sortable columns. Should be intuitive for most users.

chozo's idea of using a modal data browser showing only data folders might work.

You might also try a 2 column list box with checkboxes in the left column that indicate that an item is selected instead of using the regular selection mechanism, which can be tricky if the number of selected items is large and they are not contiguous. Using checkboxes for selection might also make tony's ideas of filtering more feasible.

It is possible to sort of fake drag and drop in a listbox, at least within a single listbox. I believe that https://www.wavemetrics.com/project/ACL_CreateLayouts does that in the listbox on the right, allowing you to rearrange the thumbnails. You might be able to use a similar technique to fake dragging from one box to another, but it probably wouldn't be very user friendly.

Another similar approach is to have a list of possible selections on the right and list of selected objects on the right and a ">>" button that when clicked moves the selected items in the left list to the right list. That's similar to the drag and drop idea but much simpler to implement. An example of this is the Data->Rename dialog.

Thanks a lot! Many great suggestions to think about and play with. I'll test few options to see, which one works well and seems intuitive. 

If you would know me, you would guess that I am thinking about raising to challenge and try to fake the drag-and-drop. Simply for the fun of it. Seems like well worth evening or two. 

You can also make a listbox with all potential candiates. And allow the user to choose each element by context menu. Choosen entries are highlighted in different colors. And of course you need a legend for the colors. Requires IP8 probably.