Yeah the code only accepts one domain AFAIK that it uses for federation and everything in the UI/API. So even if you could host it at .onion, links, images, in-browser API calls, etc would all serve from the public domain if you wanted federation. The code would need to be written to support alternative domain access with an authoritative domain for federation. In theory you could put some sort of proxy in front that just rewrote everything the server output to replace with .onion and then intercept every request to change the header so the server thought it was coming from the standard TLD, but that's incredibly fragile and a ton of work.