Alexander Suter For 7.3 you can try something like this.
import ch.ivyteam.di.restricted.DiCore;
import ch.ivyteam.ivy.workflow.restricted.businesscase.BusinessCaseDataService;
var service = DiCore.getGlobalInjector().getInstance(BusinessCaseDataService.class);
var businessData = service.find(businessCase, YourBusinessDataType.class);
Attention: This is not public API. You should upgrade to 8.0 and use the provided API.