I see a bit of contradiction here or I don't understand the question.
(1) The BaseClassLoader seems to pick up any jars in the sun base loader even though it does declare it as a parent...
(2) I want ALL classloaders to delegate downward and pickup classes for the jms.jar in the standard base class loader...
Isn't (1) saying what (2) needs?
BaseClassLoader by default delegates to its ClassLoaderDomain which then delegates to parent,
either this parent being parent ClassLoaderDomain or parent CL (e.g. bootstrap CL).
It's the ClassLoadingMetaData that controls this behavior.
e.g. for .war we change the delegation to parent-first=false, as per the servlet/war spec
If we're talking non-war deployment, this should then already work ootb.
A bit more info on MC' CL: