I have not found a way to use "Tools" with a space-separated list of URIs or any other information.
Sometimes, checking "for all selected files" causes an application to open once, with all files.
But sometimes it results in multiple instances opening, one for each file.
A way to overcome this limitation is to use what might be called "Export Tools". (my words)
Forum member DetlevD has pioneered this approach. See /t/10808/1
To summarize:
Create a Mp3tag export script that writes a script file (eg. BAT, VBS) incorporating all selected filepaths.
Use the "Export" feature to send selected files to the Mp3tag export script.
Execute the 3rd party script by clicking "Yes" when asked "Display Export file now?" after exporting is complete.
I use export scripts that create VBS scripts.
The VBS scripts formulate and run command lines for various applications.
Besides allowing multiple files to be sent to an application, other benefits to this method are that the command window can be made to not appear, and the VBS file can be deleted by the script.
For example, attached is a simple MTE file that creates a script to open the VLC media player with all selected files.
Maybe you can modify it to suit your purposes.
1Tools___Execute_Application_with_Space_Separated_Filepath_List.mte (2.03 KB)