Is it possible to sanely create an X509 Certificate in Java code without using the Bouncy Castle X509V*CertificateGenerator classes?
The ability to sign certificates is not part of a standard Java library or extension.
A lot of the code that is needed to do it yourself is part of the core. There are classes to encode and decode X.500 names, X.509 certificate extensions, public keys for various algorithms, and of course, for actually performing the digital signature.
Implementing this yourself is not trivial, but it is definitely doable—I probably spent 4 or 5 full days the first time I made a working prototype for certificate signing. It was a fantastic learning exercise for me, but it's hard to justify that expense when there are usable libraries available for free.