This bug was fixed in S11.1 SRU 17.5, which was released earlier this week.
For future reference, general instructions for updating Java separately can be found here:
To downrev, you would just specify the version you want to downrev to as part of the package name.
I also learned that the READMEs for the following patches in My Oracle Support also describe the downgrave process:
Patch 18362676: JAVA 6 DOWNGRADE INCORPORATION FOR ORACLE SOLARIS 188.8.131.52.0 AND 184.108.40.206.0
Patch 18362686: JAVA 7 DOWNGRADE INCORPORATION FOR ORACLE SOLARIS 220.127.116.11.0 AND 18.104.22.168.0
This is what I was after. Thanks Alan.