I'm not sure what you mean by filling up the positions.
There are 23 positions for Horns. 6 Left, 6 Center, 6 Right, 2 Offstage Left, 2 Offstage Right, Soloist.
If you use the transpose options, you can safely load in multiple patches of horns 1-6 and place them in unused positions. With the instruments being mostly non-linear, the actual transposing helps alleviate the small phasing issues that might come when copy-pasting MIDI (because who's gonna play in all 30 horns one by one? I know I would get lazy
), so better safe than sorry.
But once you fill them up, you can do whatever you want. Aleatorics? Call-and-response trill bursts from all over the place? It's all as easy as just playing it in. Imagine having up to 30 (or 23 per room, rather) players in front of you, spaced out however you want and being able to tell each one what to do.
Is it correct that horns are the only instrument not available as solo?
I'm not sure what made you think that. If you could point it out, I can make the post more clear.
Infinite Brass is a collection of solo instruments - there are no ensemble patches. You create your own ensembles by loading in whatever the amount of instruments you wish, positioning the across the room as you with with a click of a mouse, then just playing the lines in. While playing in an entire unison surely sounds fine most of the time, you get the best results when you either play in each line on its own or edit the MIDI if you're punching in a unison/copy pasting to at least change the velocities, note timings and re-record/edit CC1.