Even if it was blocked by the implementation, the second it hits your local machine is the second an end-user can capture it (either by recompiling the plugin, the base software, hooking DLLs, accessing the heap directly, with screenshots or even with a Polaroid).

The only advantage of OpenPGP/GnuPG over XMPP I can think of is it supports multiple end-users via merged groups out of the box. With OTR it's harder to logistically organize multi-session key-sharing. And if one of your machines is compromised, you can do a PKI revoke via CRL (or whatever it's called in GPG, slipping my mind at the moment).