6 blocks/hr * 1048576 * 8 = 50331648 bits/hr
50331648 bits/hr / 3600 seconds = 13981 bps = 13.981 kbps (with no additional overhead)
For 7 blocks/hr, 16.311 kbps (with no additional overhead)
These rates exceed conventionally available AX.25 packet radio / TNC throughputs for HF and VHF ham bands (generally 300 baud FSK for HF, and 1200 baud AFSK or 9600 baud AFSK for VHF/UHF). Either use of higher frequency bands or a custom modulation format that would tend to have a bandwidth exceeding amateur radio rules in most ITU countries on HF, VHF and UHF frequencies would be needed. At that point you might as well just set up a 900MHz, 2.4GHz or 5GHz 802.11 link.
Yeah it looks like the full blockchain over HF is a no go'er. We'd be best supporting the Bitcoins in Space project -
https://bitcointalk.org/index.php?topic=334701.0 - Or in setting up a project to build a cheap WiFi mesh-networking router that can hold bitcoind. As even the public PMR446 UHF band only permits 3.6kbit/s per channel. Although possibly maybe we could use the unlicensed 900MHz 33cm UHF band for a custom mesh-network with better propagation than the 2.4GHz 802.11n. There's off the shelf license free 802.11g 900MHz 33cm UHF equipment like this -
http://www.alvarion.com/products/product-portfolio/breezeaccess/breezeaccess-900 - and this -
http://www.xgtechnology.com/xMax-articles/xmax-system-overview.html - That we could attach a cheap lightweight nettop too to hold bitcoind.