The hardware video encoder in the Xbox console, at least the early Conexant model, does support true 240p progressive 15 kHz output. Unfortunately the Microsoft development kit (XDK) does not support anything else than interlaced 15 kHz video modes (meaning 480i / 576i). And because all emulators were made by using that XDK kit, nothing displays correctly. This is the reason why I abandoned the whole idea of using Xbox for old school gaming and switched over to Advance MAME in about 2002-3...
I think it is possible to output 240p 15 kHz out of Xbox, by some sort of tweaking that TV encoder registers or something like that... but since nobody cares, for all over those years..., it is very unlikely to happen.
It is a shame, because the closed platform is always much better than having to rely on a mess like generic PC.