The HackRF or similar SDR is useful for creating a prototype, because they work across many frequencies and can modulate and demodulate essentially any modulation scheme in software.

Once you know that it's, say, 9600 baud FSK at 433MHz, you can buy a transceiver IC for a few dollars that can send and receive that frequency and modulation, and drive it with a microcontroller.

I wouldn't be surprised if we start seeing such devices on AliExpress for $10 within a year or so.

Can you comment or recommend any resources on the legality of owning a PortaPack H2 in the US? (eg. is an Amateur Radio License required?)