Thanks guys - I'm considering just using a button and calling GetOpenFilename when it's clicked, but we'll see how that pans out... :/
In either case I have a combobox which is supposed to list each file you've selected, but of course I don't know how to tell it to change the list either 
What I'd love is to do something like:
Code:
EditPropValue(...){
select ( propid ) {
case PROPID_MYBUTTON:
// Open the dialog and get the file path back
...
// Access PROPID_COMBOBOX and add my new file path
...
}
}
Presumably I could store a list of files in editData and just keep removing the property and creating a new one with the altered list as its default?