The issue turned out to not be with MythTV, but rather MySQL as the result of a system configuration change. Recently /tmp was switched over to use tmpfs and things appeared to be working correctly until this issue surfaced. However, some digging around the system logs to troubleshoot the scheduled recording issue revealed the following in /var/log/mysql:

[ERROR] /usr/sbin/mysqld: Incorrect key file for table '/tmp/#sql_425_0.MYI'; try to repair it
[ERROR] Got an error from unknown thread, /build/buildd/mysql-5.5-5.5.31/storage/myisam/mi_write.c:226

The solution was quick: change tmpdir in /etc/mysql/my.cnf to use /var/tmp rather than /tmp and the problem was resolved.