Should be easily doable by adding a app->setApplicationName call to qt/main.cpp and qt5/main.cpp . What I wonder is if we want to use "Pineentry" or simply "GnuPG" as name, as the end users might not understand that "Pineentry" means as it's mostly an implementation detail.
Check out the GTK version which scans /proc for the process to find the command line. Very handy for ssh sessions.
as far as I understand both the Gtk and Qt implementation are using pinentry_get_title which does the /proc stuff, but this is only on Linux. On Windows, pinentry_get_title will return the value set in pinentry_init, in our case pineentry-qt or pineentry-qt5.
Maybe the title should be "Password - Kleopatra" (or similar) if the operation was triggered by Kleopatra.