Stream ciphers can operate without adding any extra delay and are considered plenty strong.
A single hard drive shipped (or hand-delivered) to each transmitter location ought to last a decade.
It would be an interesting exercise to design an encryption and compression protocol that has a high bandwidth/high latency link and a low bandwidth/low latency link working together.
Well, then it isn't an unbreakable cipher anymore.
It's only as strong as whatever encryption you used to transmit the pad over the clearnet. If you were using a one-time pad to do that then you used up the pad in the process, so you could've just told the reciever to transfer the pad from their internet-decryptor to their HF-radio decryptor.