The short answer is "no", that is not possible.
The backup is intended to restore the complete setup of Aquamail after a hard reset, incident, or on a new device. Besides just the options, it preserves account passwords, and that is the reason why it is encrypted and password protected.
Since the overarching paradigm of Android and its apps is that you are not editing configuration in an editor (or command line) as it can be done, say on Unix or even on Windows, I believe that is the reason (or at least one of them) why there is no direct access to the app setting in a text form.
Being a computer enthusiast myself, I see the rationale behind what you want, and there were moments when I would have probably preferred editing an XML for AM's configuration. But since there are very few people who would ever have used that even if it were available, I do not expect it being implemented. (And I wouldn't even hold it against the developer: it is much less important than many other things in this case.)
PS.
I should mention that when I had to edit Thunderbird configuration manually recently, when I needed to change the order of the account, I was sincerely surprised why that is not implemented via a menu... (And if you are familiar with that, - that XML is implemented in a very convoluted way.) And that's despite many users asking for the possibility to change the order of the accounts in Thunderbird.