Could be too many cooks spoiled the cast128 broth whilst
rearranging sys/opencrypto, and moving (or using) crypto transforms
into (or from) sys/crypto/*.

But sys/crypto/cast128/files.cast128 defines "cast128", and both
IPSEC_ESP (sys/netinet6/files.netinet6) and opencrypto
(sys/opencrypto/files.opencrypto) list dependencies on that
"cast128" attribute. So it looks to me like your config should work.

Can you send me a copy of your kernel-config file so I can try it out?

thanks
--Jonathan