I've not seen Service Notes for the SI-24. Roland is somewhat stingy about releasing service info for legacy(read 'unsupported') units, so we'll need to wait for a renegade with Roland access to let one go. Same deal for the VS-2000CD. Allegedly, Roland Australia will sell Service Notes to end users, but I can't substantiate this.
The only way I've found to get everything working is via an older Mac running OS X 10.4.11 and a Presonus V-Fire. Ins/Outs, faders, pans, etc., all work in REAPER using a custom keymap. Without a proper R-Bus connection, the IN/OUT doesn't work at all.
I know you're interested in the DIF-AT and not the V-Fire, but look over the posts by "Alvaro"-- he has the V-Fire and a 2480 working in OS X 10.11.xx(El Capitan)-- but I think he has only the In/Out working-- no actual 'surface' functions. The 2480 and SI-24 use the same control overlay, so what he's found may give you some insight. There is a 2480 Control Surface Guide available. If you can't locate it, PM me for a copy. Look for the R-Bus Guide as well. Pm for a copy if you can't find that either.
FYI, the SI-24 phantom power is only 15V, so some condenser mics may not perform as anticipated.