iLogic InputListBox is too simple to meet your requirements.
But you could use .NET object ListBox that has property SelectionMode to setup multiselection modes.
http://msdn.microsoft.com/en-us/library/system.windows.forms.listbox.selectionmode.aspx
Cheers,
Add-in is a good solution. You may also consider combination of iLogic rule with the DLL. You may implement any functions in the DLL and call them from your iLogic rules. It is similar to external rules but these external functions are compiled. See section "Advanced techniques to write iLogic rules" in the Inventor wiki-help.
Cheers,