Yes, the workaround is to use a pinentry wrapper script that sets the value back to the correct one and then invokes the real pinentry.

it seems more sensible to me to not pass DBUS_SESSION_BUS_ADDRESS unless explicitly configured with an option

My proposed solution is to add a config variable pinentry-ignored-env to gpg-agent which specifies a comma-separated list of environment variables which should not be passed from the client to pinentry.