MattGSX

If I remember correctly, that low D you're talking about is the false fundamental (M3 above standard fundamental) for the Bb side (I could very well be wrong, however).

A couple things:
1) If you look at Classical/Early Romantic music, you may see horn parts that are high, florid, and seem more appropriate for the trumpet. Generally, these would be written for a natural horn pitched in a higher key (horn in c/d) without a crook. These smaller instruments have a more pronounced higher range, while a crook could be added for chromatic tones/lower range. With the standard double horn/horn in F, these notes are much harder to produce cleanly.

You can, however, write for two "horn" instruments that are "sometimes" more standardly seen. One is the horn in Bb, and the other is the descant horn.

Horn in Bb - It's exactly how it sounds. I'm not sure how transposition works, but traditionally, the tone is not as desireable as the horn in F, but since it's pitched a P4 higher (or P5 lower... see below) the higher range CAN be more easily obtained.

Descant horn - the regular "double horn" is actually two instruments, the low horn in F, the high horn in Bb. a rotor on the instrument allows for switching between the two. For the purposes of playing older orchestral music (that would be written for a smaller natural horn), makers have developed the Descant horn, which has a low Bb and high F. The fingerings are backwards, and will generally require a "specialist", as compared to other auxiliary instruments. Like the Basset Horn (in F, which is also my favorite clarinet), it's usually used for modern performances of older music, but if you're writing for a group that you KNOW either has the descant horn, a single horn in Bb, or a high horn in F, this could be a great instrument to use. I know my school has one and I'm at the "off-shoot" campus of a larger state school (to be fair, though, our horn professor is Bruce Atwell, who has enjoyed a more-than-modest share of success as a performer and pedagog). The horn in Bb, however, is not incredibly uncommon. Most music educators recommend the single horn in Bb, since most "beginning" repetoire is written in the range more suited for this instrument.