Ideally all debug features (breakpoints, watchpoints, tracepoints &c.) will work running both forwards and backwards. To meet the standard of the "perfect debugger" from the TFA intro, I would say you should be able to run backwards, modify a function, and run forwards again. I'm not aware of any debuggers that let you do this.
I remember once debugging a thread without killing it. I've asked a customer to click on a link that would get to a bug while having remotely opened the IDE that was serving that user session, setting a conditional halt. Seen it halting, fixing the bug and saving the method with the halt removed and let the thread run. All the user saw was a long request that ended in service instead of that bug.
Executing arbitrary code in the program's context is also a regular debug feature, but I don't think many reversible debuggers allow this.
Pernosco supports this too.
"The stack of (framed) execution contexts gives a history of the computation so far. You can select any frame, view instance values in the receiver, view the arguments and method variables at that point."
So step backwards, modify a method, step backwards before that method send, and resume execution with the modified method. (Note: resume rather than restart, so the modified method has the preserved context unless we manually edit that context.)