I see the 777 is no different to the old 707 regarding access to the ATU. The 707 usually had one in the fin and one in the stbd. wing tip.
I also remember Sunair HF sets that were often fitted to smaller aircraft like Islanders for ferry flights. They had a TR and an antenna coupler, they used a trailing wire antenna that had to be tuned by varying the length. The controller had a SWR meter to use when tuning.
We had one in an Electra, with a fixed wire antenna, for a transatlantic ferry flight but it was next to useless. Fortunately the VHF coverage at high latitudes was very good so we didn't need the HF.