using hardie backer as a temporary floor?
You can walk on HB as long as you like.  If it is of the 1/2" variety I seriously doubt you could damage it.  However it does emit silica dust.  If you walked on it vigorously you could kick some up and it is really not a good thing to breathe or be near food.  You can certainly paint it.  There are waterproofing paints used for backer boards like redgard... However these will generally smell like puke for a week or two and given it's in your kitchen maybe not a good idea.  I would then go with a primer coat - which might need to be scuffed up a bit with a brush before thinset/tile.