What kind of digital are you suggesting? Voice and data both?
The Eagle project at one point proposed an AMSAT Advanced Communications Package (ACP) microwave, digital-uplink (5650 MHz) and digital downlink (3400 MHz).
Realizing that microwave earth station design is beyond the scope of most hams the Eagle team proposed making an ACP-capable earth station within reach of most radio amateurs that was thought it could be distributed along the lines of the project kits offered by the Tucson Amateur Packet Radio group (TAPR).
Radio link budgets and on-air protocols were being studied so the average earth station could have used 24 inch diameter dishes similar to home satellite television. Well, that was a goal anyways.
This Advanced Communications Package has been called 'Project Namaste' and 'Microwave Engineering Project' over the years. There was a proposal that a portion of the transponder would be reserved for analog operations so there was some chance the gear you already had would be useful, perhaps with Transverter or receive converter. You would need the ACP package to take advantage of all the advanced features of the network.
The Doppler shift at microwave frequencies would be manageable on HEO or GEO. It would be quite a challenge on something moving across your range like the ISS does.
A digital path from anywhere on the planet to the appropriate ground station is easily doable with some "documentation" of the ground stations.
This was what was proposed back in 2008 at the Atlanta Symposium: http://www.saloits.com/papers/AMSAT2008.pdf
-- 73 de JoAnne K9JKM k9jkm@amsat.org