Often, I have reason to repeat the last action I was performing in Mp3tag, hence a keyboard shortcut for this action is certainly going to be appreciated 
Thank you.
That is a good and useful suggestion, ... my vote +1.
Have you ever considered, if you create a permanent action, ...
then you can call this action by a maximum of three key strokes.
For example ...
.. having an action named "Set Media &Format#&Vinyl" ...
... which is defined as ...
Action "Format value"
Field: MEDIA FORMAT
Formatstring: 'Vinyl'
... one can set the tag-field "MEDIA FORMAT" to 'Vinyl' by typing ...
[Alt+A] [F] [V]
DD.20150418.1844.CEST
Would like to add my +1 to this suggestion.