Induced interference that can be measured (c64)

Hi there,

Just out of curiosity, I got a crazy idea. Is there any means to
detect an external signal be it light / sound / magnetism / heat etc.
on the C64? I know there isn't a proper one, and the thing is almost
digital except sid chip.

The idea here is : Input a small program to C64, run it. Run a data
transfer software on a phone and transmit data to the software on the
c64 using that covert channel. So possibly heat and magnetism is out
of scope since we can't induce those easily with a phone :)

The closest possibility coming to my mind is : inducing interference
on the POTX / POTY lines. They seem to be connected directly to the
sid. Say anything that can change the analog value read from those
cycles would be  a candidate.

I don't think we can do anything with light I guess and the soundwaves
are possibly very out of spectrum to induce anything on these analog
circuits.

What do you think?

Regards,

Nejat