On further examination, it seems more likely that the DDC channel enables a DVI display to tell the HDMI source that it's just a lowly DVI display, so the HDMI source can go into backwards compatibility mode, whatever that may entail.

Of course, all this is contingent on the assumption that the 990 is "transparent" as far as the display and source are concerned - which seems likely based on what the OP is reporting. In this light, it seems to me that the fact that the 990 doesn't transcode to DVI or offer OSD on DVI is, in fact, a feature