It seems there's some trouble with using @ScriptAssert under OSGi as per this question Not sure whether it's a question of adding another import or what is missing really.
So thanks to Marko's work we ended up reworking the entire approach for evaluating the script constraints. There's now a new SPI (centered around ScriptEvaluator and ScriptEvaluatorFactory) for making the retrieval of script evaluators more flexible: one can plug custom script evaluator factories that just customize the aspect of class-loading (e.g. for retrieving script engines under OSGi) or even factories that return script evaluators other than for JSR 223, e.g. for Spring's SpEL.