simon,
you might have to get a disc hub laced to your 24" rim
ready-built 24" disc wheels are very hard to find...I was looking for 1 last year for a trike project...never found one, had to get 1 built
Got to bikepartsusa.com. I did not see a 24" wheel with dics brake hub. I just got off te phone with them for problem. Out of curiosity, I asked about a 24" wheel with disc hub. I was told they do not stock them, but they can build them. Total cost he quoted was $75.00 You should be able to get about anything built by a ton of wheel vendors/makers. The problem is cost ! Hope this helps.