Why is there no inbox support? And what exactly are the limitations?
You answered your own question
If not on BES, you have to use an app like PGpgp which is limited by the BlackBerry API. As you said, no direct inbox integration, the app uses the Share function or you can copy-paste the content for decryption.
I haven't tried PGPGP myself but it is a decent option if you are going to make use of PGP with people.. you can launch the app from your email in the hub, and can compose your email within the app and send in the app.
You'll have to manually import and exchange the keys, but once the initial setup is done it doesn't look like it would be a huge hassle.
You just can't have direct hub integration, your email in the hub will be encrypted garble, that's my understanding of how the dev explained it.