I've been thinking about adding a little extra CMC or Tylose to commercial fondant to make it a bit stiffer and hopefully more manageable for covering cakes. I'd like to be able to apply it in a thinner layer while still getting a good, smooth finish but don't want it to turn out like concrete! Has anyone tried this? Any advice much appreciated.

I have done this in replace of gum paste to make figures but I would not suggest it for covering a cake. Your fondant will dry quicker and you will probably get a lot of cracking and will not get a smooth finish on your cake. Because of the quicker drying time you have less time to get it on your cake. It will not make it more managable.

Thanks for your reply, kakedecorator!  Since my original post I've tried this and it has actually worked quite well.  The secret seems to be not to add too much, and the amount you need depends on the softness of the fondant you're starting with.  As a guideline, if I'm using SilverSpoon brand rollable cake icing (common here in the UK), I add 1 level teaspoon of Tylo powder per kg of icing. So long as I don't let it hang around for too long it stays workable and I've not had problems covering my cakes smoothly.