Interesting question, has been covered somewhere before, can't recall where...
There are regulatory issues and practical issues;
Regulatory;
I am sure the FAA does not have a problem with it if:
-Student already holds a US license for category and class, eg single engine land. You would simply be a more experienced passenger with your student being PIC. You would only be able to instruct for a license or rating which is not recognized in the US, therefore a foreign license, eg JAA PPL/CPL
Practical:
Insurance companies in the US are NOT going to be happy, to use an understatement, if you are instructing without holding the required qualifications.
Remember you will be flying a N-registered aircraft in US airspace.
Question is also; how are you going to be legally employed (visa or otherwise) to flight instruct without holding a US CFI?