mRTK was available for some years in Europe. I was told by a Deere distributor from one country that it is a yearly subscription. 

With a Deere base multiple devices can replace the Deere radio and use NTRIP. Intuicom modem, our RTKonnect wifi client, and Digi's little blue thing all are able to replace and emulate a Deere radio.

The only thing required is a Deere base to tap into to make it available by NTRIP. Our freeware caster will single baseline Deere data from a Deere base. I'm not sure which other caster programs will or will not do that. You can Tee into the Tx and ground wires in the 4 pin radio connector and read the NCT data to send to a caster while still leaving the radio operational. I'm really surprised many Deere dealers haven't done that to make their existing bases usable to more than one customer type and get over line-of-sight issues.

Years ago, Base-n-a-Box had GPS RTCM2 to iTC /Gen2 NCT message conversion. The guy who started that is the same who also helped Digifarm get started and sells them their Geo++ caster software and used to sell them the Chinese/Trimble bases they used a lot of. Digi supposedly has that software now. Ask Joe as he owns part of Digifarm.

RTCM2 converted to old NCT (gps only iTC and Gen2 messages) is easy enough. That is straight forward.
RTCM3 converted to old NCT takes a lot more work as the data must be reconstructed similar to how the Deere makes its calculations.
conversion to current NCT (GPS/GLO SF3000/6000) is tricky as Deere built in multiple mathematical puzzles before you can reconstruct the data. I keep telling myself I will get back to that and finish the last 2 bytes of data but it's been put off for a good while now. I fully expect Deere to again change the NCT format with the release of the SF7000 as they will be adding all constellation tracking and data at that time. If they do change it all, then all of my past decoding work would be for naught.