It would not take more than a day to reverse engineer the garmin crossfill protocol. You would need an ARINC protocol analyser. Then, potentially any flight planning program could do it.
It would not be legal, but would be a super feature.
It's probably been done, but it's unlikely to be advertised
Being able to plan your route and then view your progress over that same planned route is a major advance in safety, as it eliminates the most common human errors. It's a shame that we live in such an anally retentive world.
Mind you, being able to connect a
keypad to these units would go a long way...