If backup/restore doesn't bring it over (surprises me slightly), then I can think of a possible way to bring over the Workflow History list as a separate step. I'm not sure if any links/references to it will still be in place though, you'd have to try it.
Anyway I think you might be able to bring it over with the Content Deployment Wizard (I'm the creator). You'd need to edit the SPDeploymentWizard.exe.config file and add the following key:
<add key="ListsNotForExport" value="Cache Profiles, Content and Structure Reports, Converted Forms, Long Running Operation Status, Notification List, Quick Deploy Items, Relationships List, Reporting Metadata, Reporting Templates, User Information List, Variation Labels, Workflows, Workflow Tasks, **Workflow History**, fpdatasources" />
This is the 'default' set of lists hidden from the treeview - you can remove 'Workflow History' from the list above, which will make it selectable as an individual list. This should enable you to pick up that individual list and export/import it.
Not 100% sure this is the real problem you should be trying to solve though, seems an unusual situation.