Re: I use my own encryption system
You could prove this stochastically, by plotting all the 1-digit numbers against how many digits into pi they can be found, then all the two digit numbers, all the three digit numbers, and so on.
As the number of digits gets larger, and thus the sample size for each round increases, you could take intervals and plot how many numbers are found in that interval, and you'd almost certainly see a poisson distribution.
e.g. for ten-digit numbers, you'd see that a small number occur very early on, with the number in each interval (let's make the interval size 10,000 digits, you might get a better curve with larger intervals) increasing, then falling off. Some sequences won't be found until very far along.
The interesting thing to see would be where the peak of that curve falls, compared to the mean value of what you are trying to "encode". I have a hunch that it's going to be a bigger number (e.g. a ratio of higher than 1), and with each round, where you add a digit, that ratio is likely to increase.
If this is the case, then this is a demonstration (if not a formal proof) that as the length of what you are encoding increases, the position of that thing within the digits of pi tends to infinity.
A real mathematician might be able to turn this into a formal proof. The last time I studied any maths was at A-level *mumble* years ago.