However, I am queasy about installing external programs to get “simple” split-gpg-like functionality. Is it not possible to simple make pass, pass-tomb, etc, make use of qubes-gpg-client-wrapper and make use of my existing split-gpg setup?
I don’t know how these which you mentioned work. I manage passwords very simply: one password for everything. Wait! Keep reading.
I have a simple bash script that combines mathematically the username, domain and a password, then outputs a complex ASCII string with desired length. So, considering user and domain combination are always different, it makes possible using only one password, resulting in different actual passwords. No need to store secret data anywhere.
Run that script in an offline disposable, copy the password to the qube you want, done.
Simplest example how you can do something like that:
Mine is more complex and outputs upper and lowercase letters, special chars etc. You can create your own algorithm.
The only inconvenience is for systems which require periodic password change, as you would have to remember new pass for each change. But even that is simple to manage, e.g. pass=mysecretJANUARY25.
I haven’t shared this in a guide as it is not really Qubes specific.