Easiest way to fake the call home is to put the address name in your HOSTS file and equate it to 127.0.0.1. It then depends on how it deals with making a connection but getting no answer.
For RSA you need a big number library. There are only 2 or so in common use, so if you make IDA sigs from them you should advance a lot. RSA is very simple math done with very big numbers. Look here : http://en.wikipedia.org/wiki/RSA_encryption