I don't think working on getting compatible with the current Live client is necessarily pointless. Twice I updated the HoT patch to the point where you could log in/zone in with spawns and inventory, purely to provide a more current starting point on the off-chance that a HoT client was released on Steam, but as things turned out, it never was. (So I suppose in that sense, it was pointless
What stopped me going any further was the fact that the inventory slot numbering was all changed to allow for variable size bags and I didn't feel like putting in the time to write all the encodes/decodes to get item handling fully implemented. (I did enough so items would show in your inventory, but moving items around, clickable items etc was not implemented).
Instead of a single number indicating the slot, e.g. 1 - 30 for personal inventory, 400 - 404 for tribute, 2000+ for bank slots etc, the new system introduced in HoT uses a 3 part numbering system, the first number indicating for example, whether it is personal inventory, tribute, bank slot, etc, the second number being the slot within that category, and the third number being the sub-slot (i.e. if the item was inside a bag).
So yeah, inventory manipulation would require the most work to get a working VoA client.