I have heard in the past someone who said having done a transaction with Bitcoin fully offline. But he had a ham license and probably his partner was linked to Internet.

Unfortunately as far as I know the satellite to sync the Blockchain with the setup of Blockstream is only one-way, which is receive-only so we can't send transactions afaik.

Even if it were possible to send to a satellite or through any kind of radio waves it would still require anyway an electronic device with most likely a battery (although it could be a solar setup) so my guess is you would be better off buying a 20000mah battery.