No.
At least, not if the person building the ROM knows what they're doing.
From the docs (emphasis mine):
Each key comes in two files: the certificate, which has the extension
.x509.pem, and the private key, which has the extension .pk8. The
private key should be kept secret and is needed to sign a package. The
key may itself be protected by a password. The certificate, in
contrast, contains only the public half of the key, so it can be
distributed widely. It is used to verify a package has been signed by
the corresponding private key.
If your device has an unlockable boot loader, you could build and flash your own ROM, using a key pair that you created yourself. That way, you'll have the platform private key to use for your system app.