Why should I bother to make a particular test (which you do not know if I have or not) if it is pointless: the developer says,
then this is it.
I can live with that easily, be it in a portable installation or not.
And even if I test I find out: yes, the cfg file has to be writeable. And now?
I think that you somehow try to install some kind of separate user administration, only valid for MP3tag: some are just users with limited access to certain functions and others are the administrators.
MP3tag does not claim anywhere that it supports such an approach. So where is the limitation?
I still wonder what the purpose of the blocked cfg file should be? That you modify the files being read? The language?
I think that the plain editing functions in MP3tag can cause much more havock in a collection than any modified cfg-file. So how do you block modifications to the music files if they do not make sense?
Or do you use MP3tag as viewer? Then have a look at the subtitle: the universal tag editor. And editing means modification.
Coming back to an alternative: if you do not trust your plain users (which you should as you have to as long as you use Mp3tag) then create a cron job (or a repeated planned task) that copies an original portable MP3tag installation into the production environment. There may be a little time lag if someone has messed up things until the next cron job starts but that is a close as you can get.
And just a reminder of the current state of this thread: it has already been classified as "no bug" - it all works according to design. The limitation that you want to get removed, and I still think this is the case, lies in your approach but not in the Mp3tag features and functions.
Ah, just one thing: yes, one could claim that if one does not modify anything then why write a cfg file? To be honest: I do not know why MP3tag attempts to write but I bet the developer has a profound reason as he designed it that way. Without more knowledge about the internals we have to take his word for it that it is necessary. And that's it, no limitation.