Figured this would end up being something trivial/inane and it was in the end.
Setting mapred.child.java.opts
thusly:
SET mapred.child.java.opts="-Xmx4G -XX:+UseConcMarkSweepGC";
is unacceptable. But this seem to go through fine:
SET mapred.child.java.opts=-Xmx4G -XX:+UseConcMarkSweepGC;
(minus the double-quotes)
sigh. Having better debug options/error messages would have been nice.