On the one hand, yes, I would say that it's possible to set up software on a laptop or tablet PC that would mimic a fancy universal remote. Connectivity to the system would be necessary, but that coule be accomplished somehow I'm sure. In a way, custom integration systems like Crestron are at least similar to PC's.

On the other hand, no, I would say that it is not practical. The hardware cost (both the laptop and the hardware required to interface with the system somehow) would rival the most expensive remotes from Universal and Pronto, and the labor required to achieve a user friendly interface would likely be significantly more than setting up either of those would take.