I have used chaff quite a bit in Hawaii as a mulch but never used in mixed into a raised bed. I would supposition that it will add to the floculation of the soil and in the long run will add to the organic matter in the but will take extra nitrogen for the microbes to digest the carbon component of it. It is a little hydrophobic (repels water) not bad but you need to be mindful of this. The coffee cherry is on the other hand quite rich in nitrogen and is used more as a organic nutient supplement for all kinds of orchards in Hawaii not just coffee. I don't recall whether the Hawaii coffee association has any run down of the different parts of coffee as to nutrients or not. I think Rodale has it listed but that has been many years agao and I can't give you any concrete off the top of my head.
Good for mulch and might be OK for your use but I feel personally if I was to use the combo half horse and half chaff I would do it over the fall thru spring fallow period and then plant your crop. Although in zone 9 you won't have much of a fallowing period.
Just some thoughts to think about.
Good Luck and Happy Growing David

In another forum, I wondered about its use in an organic garden as coffee is a very high pesticide use crop. There are some organic growers, but the vast majority.......;-(

As to chemical use in coffee again I can only speak of Hawaii and in general terms. Not much is applied directly to the foliage. Most growers I knew and still know used glyphosphate (Roundup) for weed control if bare ground and grass middles for erosion control many mulch and have mown grass middles. Ants sometimes were a problem and most many growers dealt with the perimeter of the property with amdro. It is not allowed on food or any consumed product such as coffee. Many use chemical fertilizers but it was also common to use the chaff, cherry and even the leaf, and shredded pruned wood in the orchards for mulch and slow release fertilizing. The small farmers in third world countries don't have the means of money to use much chemical of any kind. It is much more subsistance farming than you may think. The larger ag concerns on the other hand would be a different matter and would use whatever brought them the best bottomline which may well include several chemicals as needed. Since it is a food crop there are several materials not allowed for use. This would depend on a given countries laws and regulations of course.
I would myself use it at least as mulch or in composting if it was readily available to me.
Just some thoughts from a one time coffee orchard manager in Hawaii over 15 years ago. (my how time flies).
Happy Growing David

Pesticide laws in other countries are not equal to US regulations regarding food crops. Many banned products are able to be (and are) used widely. But, anyway....I was just wondering if residues in the chaff would be a problem.

To go back to the OP's question about using half semi-aged horse manure and half coffee chaff for raised beds, I would say that making it half manure and a quarter each of the chaff and some kind of soil (potting or top) might be a better mix. That said, I have used only compost (a year old) in a large pot, and grown good plants in it. The thing to realize is that the mix will keep 'working' and will diminish over the summer. Your plants may descend below the level of the edges of the beds. You will probably have to keep adding more. If you are growing annuals, this won't be a problem, as you can just add at the end of the season as you remove the old plants. If you want to plant perennials in the raised beds, then you might have a problem, as few of them respond well to having 'dirt' piled up over their crowns, and lifting them and applying more dirt under them isn't really an option, not if you want them to keep growing and flowering. I would fill the bed to the top of the edge, mounding it up in the middle, if it were me.

I would also, since it's free, take as much of the chaff as I could and compost it with the manure and whatever else I could get, to have a good supply of compost for the future.

It would seem from what has been said, that residue from pesticides is likely to be minimal. If the chaff is composted, then any residue should be degraded by the time the compost is used.