You could try the vapour sterilisation method when replating. See here:
http://www.orchidboard.com/community...seedlings.html
I've just done my second replate for my Bulbophyllum orectopetalum seedlings. You just adjust it to 2-2.5 hours instead of the 4.5-5 for seed sterilisation.
I didn't get any contamination on the first replate. I only did the second two days ago, so maybe too early to tell but it looks good so far.
It's worth a try anyway. Just use newly opened bleach. If it's been open for a while the chlorine starts to dissipate and this is what you're relying on to kill the contamination so it needs to be fresh.