HeXen doesn't "patch the BIOS for HD video". It walks you through enabling HD in the eeprom, after you have enabled NTSC.
If your XBMC doesn't have the HD options available then your Xbox isn't in NTSC mode, or you don't have a HD capable cable connected.
You can't enable HD with a composite SCART adapter - you need a component or a genuine HD SCART adapter.
S-Video doesn't have lower quality than SCART. SCART is a connector that can carry many types of signal, some better than S-video, some worse.
Guessing about an Xbox being able to output 480p over SCART on the basis of what a WII can do is, frankly, barking mad.
So, get a proper HD cable, switch your Xbox to NTSC, enable the HD modes from XBMC, then come back with any residual problems you may have.