I bought this thing on eBay as a dead curiosity, but was determined to fix it. So, after replacing a bad fuse and making extra sure that all cards were seated onto their connectors, it worked.
I tried a CardBus USB card on it, and it worked perfectly the first time (under OS 8.6). It wasn't until I got a "real" 2400c that wouldn't use CardBus that I learned that it was supposed to be disabled. IBM made this thing completely CardBus capable (with the TI chip and special card cage), but Apple wasn't ready to support it for whatever reason (maybe 7.6 just didn't like it), so they had IBM disable it on the shipping machines.