I'd be really tempted to move this into the ZAP addon - not sure Zest needs to define default variable names so it probably makes more sdence being in the add-on. The way I've done it for auto generated anti csrf token handling is here: https://code.google.com/p/zap-extensions/source/browse/branches/beta/src/org/zaproxy/zap/extension/zest/ExtensionZest.java#461