In the workflow customizing (yes there is such a thing!) you can define substitution profiles, task classes and the link between them, you can define a profile for the travel management assign the approval task to the class you defined.
when creating the substitution - assign the travel management profile and not the general one and the subsisting user will see only the travel management tasks.
see Defining a Substitute During a Users Absence - SAP Business Workflow - SAP Library