How can I use the time:xxx functions in custom processor configuration fields?

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If you look at the existing stages, for example, the HTTP Client stages, you will see how they use the TimeNowEL class to do this. You need to add elDefs and evaluation attributes to the @ConfigDef annotation on your config parameter, like this:

evaluation = ConfigDef.Evaluation.EXPLICIT,
elDefs = { TimeNowEL.class },

You'll also need to add code to the processor to explicitly evaluate the config parameter:

// Field declarations
private ELVars configVars;
private ELEval configEval;
// Config property field name
private static final String CONFIG_NAME = "myConfig";
...
// in the processor init() method
configVars = context.createELVars();
configEval = context.createELEval(CONFIG_NAME);
...
// Per record, or per batch - however often you want to do this
TimeNowEL.setTimeNowInContext(resourceVars, new Date());
String evaluatedConfig = resourceEval.eval(resourceVars, myConfig, String.class);

Finally, since TimeEL is not a part of the official API for custom stages, you will need to build the stagesupport jar from source in the main datacollector source tree, and include it in your custom processor. See the custom destination tutorial for details - that destination uses commonlib for CSV formatting; you should be able to do something similar with stagesupport.

Be aware that stagesupport is not an official API; it can change without warning from one release to the next.