This technique http://imgur.com/a/dCvS5, shows how is easy to do permanent markings in stainless pot. But what about aluminium ?
Does anyone know if it is possible (and safe) to use it in aluninium pots ?
Electro-etching works with all metals, including Aluminum.
Often, the makeup of the electrolyte is changed up to give better or worse results with different metals. In this case, you don't actually need an acid as per the above - just a salt water solution works fine for aluminum.
However, please make sure to do this in a well ventilated area as electro-etching with table salt (NaCl) will result in small amounts of chlorine gas being produced, and that certainly is not good for you to inhale.
I would recommend you test on the bottom of your pot to be sure you have your process down before etching the interior. While it's a relatively straightforward process, some experimentation in a non-visible area will help you get an idea of how long it will take to get an etch of sufficient depth / color.
Once you've etched the pot, boil a full pot of water for 20-30 minutes to re-establish your aluminum oxide layer in the areas you've etched. Note that this may make your etch a little harder to make out, but that's unavoidable.