The comments that are saved while REJECT should be saved in the workflow container under a container element named "ACTION_COMMENTS". can you check the workflow log in swi1 Or swi2 and inspect the workflow log?
>>>>>Can I intercept this? the question is what do u want to do with this and what and where do u want to intercept?
if your objective is to show these comments next time on the same work item when it comes to the user's inbox in Fiori then yes you can do it but you would have to enhance the oData service for Travel Expense Approval and then check the workflow container for these user comments. it can be done but depends on what u want to do. technically things are possible 
Best regards
Ashish