We try to use an expression like ${myService.myMethod(param1, param2)} in ruleVariablesInput of the businessRuleTask. Looking in the forum, there was a similar bug with the same exception almost 2 years ago: http://jira.codehaus.org/browse/ACT-428 I think the same patch could be applied here.
That should definitely work, so it could be a bug. Can you create a JIRA for this and attach a unit test like described here http://www.jorambarrez.be/blog/2012/09/24/how-to-write-a-unit-test/ ? You can assign it to me or post the JIRA number here and I'll take a look.