I'm a semi professional who has played in a Wind Symphony for 20 years and then Orchestras for 15. Currently I am principal and have been for the last 5 years. As principal the director looks to me to help him with Horn placement within the section.
Typically horn sections are lined up assist,1,2,3,4 left to right. Its important to have this configuration as your bell faces into the next chair. This helps the player to your right hear you well to balance dynamic and intonation. TYhis also gives the principal greater control over their section.
Using
redbirdstudio.com/AWOL/stage.html
as reference, I find its better if the section is stage Right. This places first somewhere just right of center. It helps to have principals near the center so the director can find you quickly. I also find that Up-Center works pretty well, if not placed directly in front of a wall. Horns need at least 6 feet between the bell and a hard surface otherwise the sound emanating from the bell reflects back into the bell causing issues with playing.
For Brahms Second Symphony I placed the horns in a square pattern. First and second in front with third and forth behind. In Brahms and many other works of that time that carried four horns the horns are actually two sections of differently keyed instruments so being in a line is not required. We were also Stage Right.
Another important consideration is who are you mostly playing with. If its the Woodwinds you should be located near them. Personally I'm happiest when I'm in the Up_center and directly behind the principal bassoon and clarinet. If its a big brass blow, lining up as a brass ensemble with Horns stage Right works well.
Once for Bolero I had my assistant sit at first then, 2, 3, 4 and I sat to the right of fourth. I did this because the flutes I play the solo with were closer that way, I could see them and coordinate the solo with them better.
Some things to avoid.
1. Do not place horns right in front of a curtain. The curtain will very will effectively suck any sound you produce into it.
2. Do not place horns 2 feet from a wall. Try it your self. move up against a wall and try and play loud. Careful though you may injure yourself.
3. Do not place a horn next to the Bass drum. I cant tell you how many times this keeps happening, but when it does if Im playing fourth and am up against a bass drum I will not play. Sit to the left of a bass drum with your bell facing the drum then sustain a note, while someone beats the drum at full force. Each beat travels through your horn which acts like an amplifier, takes the bass note and shoves it into your buzzing lip.
4. Do not place horns such that their sound is driven into some alcove. Another sure way to lose most of your horn sound, (unless you sitting in the alcove).
5. I don't think stage Left is an effective location for horns, but can work if there is a wall (more than 6 feet) or shell behind you. Shells btw are very effective and I encourage their use on any stage.
A note about reflectors. They do not work. I did research on them and while some hornists say they work, sound engineers say they do not. What is working with a sound reflector is you can hear your sound reflected, but only part of it. Most of the character(high and low overtones) of the sound proceeds straight through the reflector. You have effectively taken your beautiful tone and split all the low and high frequencies off.
As for resources, you are right there isn't much. Most of what I found was from discussions such as this one, but a searched horn matters and found an article of interest:
hornmatters.com/2009/09/horn-section-placement-on-the-stage/
fwiw, Hornmatters.com is a great resource for Horn players.
Eric Anderson, Principal Horn with the American Philharmonic - Sonoma County
