I do not understand what you mean with ...
"filtered out when Alt-F1 is used anyway", ... please explain.
The following characters are not allowed to be used within a filename ...
$char(124)$char(92)$char(63)$char(62)$char(60)$char(58)$char(47)$char(42)$char(34)$char(31)$char(30)$char(29)$char(28)$char(27)$char(26)$char(25)$char(24)$char(23)$char(22)$char(21)$char(20)$char(19)$char(18)$char(17)$char(16)$char(15)$char(14)$char(13)$char(12)$char(11)$char(10)$char(9)$char(8)$char(7)$char(6)$char(5)$char(4)$char(3)$char(2)$char(1)$char(0)
... but Mp3tag's dialog "Tag - Filename" does not remove all these characters.
For example ... Mp3tag accepts the backslash as a valid character for the filename.
Further on ...
$validate($char(124)$char(92)$char(63)$char(62)$char(60)$char(58)$char(47)$char(42)$char(34)$char(31)$char(30)$char(29)$char(28)$char(27)$char(26)$char(25)$char(24)$char(23)$char(22)$char(21)$char(20)$char(19)$char(18)$char(17)$char(16)$char(15)$char(14)$char(13)$char(12)$char(11)$char(10)$char(9)$char(8)$char(7)$char(6)$char(5)$char(4)$char(3)$char(2)$char(1)$char(0),'#')
... creates a preview of ...
########e a.mp3
... which is a rather weird filename ...
and it demonstrates, that the function $validate does not validate all forbidden characters.
About invalid characters look there ...
[F] $validate() broken?
I do not believe that Mp3tag accepts the / slash character as part of the filename.
Try it out in Mp3tag Listview column to insert a slash into the filename, ...
then you will receive an error message.
The / slash character is not registered as an invalid path character ...
it is the valid path separator on Apple Mac and UNIX systems and also used for web addresses, ...
and for example, ...
someone can use the slash character as the path separator within the Explorer address line.
DD.20150321.1845.CET