I don't think it's the fediverse that's quite the problem. Two issues that I can think of that makes this harder to achieve:

1. You'll have to fuck around with router settings to forward the incoming traffic to that piece of hardware.

2. You'll have to have a domain, and fuck around with dynamically changing DNS settings because your IP is likely not static (this process depends on which DNS provider you choose to use).

To add to the complexity, you'll probably have to have some sort of a VPN because otherwise anyone would be able to see your public IP by just pinging your domain.

Point being: you can run Pleroma right now on a Raspberry Pi no problem, but doing so securely is another issue altogether.