Goggle is telling me that a 'h2o mop' is a steam cleaning device (from the USA), and if so you'd risk ruining your plastic fermentors for no real benefit.
A good soak in hot water and sodium percarbonate then a light wash with soft cloth is all you need to do to clean them well, why bother with anything else and why use something that would damage them?
Having said that some people do use their plastic fermentor's as a 'bucked of death' to boil water, wort and stuff in so, maybe you could get away with it, I just don't see the point.
I use a little steam device to clean the threads around the tap area.. Called a steam matey.. While i cant put my hand on my heart and say that its actually sanitising it sure does make me feel better about the threads..
After saying I had no idea what this device was, I discovered the flash little red machine in my wife's cleaning cupboard (dark territory) was in fact what the discussion was all about. I have been having a bit of trouble with infection so I gave the steam mop a trial in the fermenter for the last couple of brews, mainly as iamozzieyob says to clean the impossible-to-reach areas around the valves and temp. gauge that even a toothbrush cant reach.It appears to have been totally effective in removing the bad bugs and as a bonus, has lifted the patches of oxidised polyethylene off the fermenter walls which i presume were caused by extended exposure to sodium perchlorate and/or chlorine which I was using to try solve my problem. The beer is good again!