So this weekend I wanted to see if I can write this feature for this plugin, then when I got the master branch running, I say the functionality is already there. I just hasn't been released to be available on the jenkins plugin store.
With this configuration I could backup all the keys there are in the secrets directory. With some tweaking of the regex you could probably match all the files in the secrets directory.
The source code for this has been done more than a year ago. Is there any plans to make this available from the plugin manager?