We recently tried to implement the chanbara as well as foam rubber covered wooden bokken into our kendo kata practise in effort to bring a little more earnestness to uchitachi's attacks. We broke the centre piece on the chanbara pretty quickly! We also discovered that it would still be quite easy to injure with the foam covered bokken.
It did help us to adjust the distance necessary for evasion in Kata number 1 and pointed out a weakness in kata number 7 ( the uchitachi whose sword is sincerely going for the head strike will follow shitachi's head somewhat as shitachi enters so he better enter quickly and drop his level suddenly.) They were all but useless for katas #5 and #6 which have some blade to blade contact in the deflection and counter.
I think a foam covered shinai would have better resilency but would also have the same shortcomings as any rounded weapon posing as a facsimile for sword. Not bad for distance awareness, though.
Matt Rogers