I will quote from the whitepaper, it's fairly easy.

You will be regarded as a Congress member if you meet the following criteria:

1. Run a fully-synchronized node at stable network speeds
2. Freeze at least one unit (one frozen unit is 10,000 BOS)
3. Participate in voting

Anyone can become a Congress member. A node could be a server or a personal computer and the node can be located at home or a remote location, as long as network speeds are stable.

It's not going to be a substitution because the Trust Contract is based on a totally different technology. We are simply not aiming for a turing complete contracts, but we are more focused on providing the decidability.