Full length housing sounds like a great idea - I wasn't sure if that would work. So, on the diagram, it looks like the housing runs all the way to the derailleur...and the cable would be entirely enclosed from the brifter to the derailleur stop. I'll bet the builder could figure out a way to keep the cable controlled and looking good against the downtube.
I found some info on the Shimano CX70 top pull front derailleur that's made for cyclocross. It's probably the wrong chainline for the GRX crank, and so I'd probably have to get a different crank, and the CX70 might not be compatible with 11sp... - and then I'm immediately over my head as to how to make it all work, so I'll try the GRX with full length housing. Thanks.
It does seem odd to me that Shimano didn't make a top pull GRX front derailleur too, though. Gravel can be sort of like cx with all the muck that collects under the BB.
I can confirm that the CX70 does indeed work with the 11sp pull ratio. So using a 6800 lever would be no problem. The chainline issue for GRX would still be an issue, but you could avoid that by using a road style Shimano crank. If you're OK with the road size chainrings, of course.