Coming across this topic, I thought I could tell...
I have recently developed ATC-pie
, an ATC client for the FlighGear multi-player network. My last addition is a proposal for a solo training mode, which would answer the request here. See announcement here
It is still under beta, so not directly downloadable from website instructions, but I would be happy if the people interested tried it. Contact me if you would like to, or wait until release if you are patient.