Maybe you can try to record a system exclusive message from the organ into Sibelius. (Connect the organ's MIDI OUT port to your computer's MIDI IN port, start recording in Sibelius, change the registration on the organ, stop recording in Sibelius and see if it captured a system exclusive message.)
If you want to further understand how to modify or create your own system exclusive messages for these registration changes, I or other friendly MIDI nerds that hang out in this forum may be able to help explain things. Take a look over the information I found below and feel free to ask us to explain any parts that are confusing.
Sibelius 4 Reference:
Starting on page 18, the section "Flexi-time" (real-time input) describes the recording function.
* Page 20 describes MIDI data is recorded including "controller", "pitch bend, modulation, volume, etc.", but it's unclear if that means system exclusive is included in the kinds of things it can record.
* Page 22 describes recording options to "Keep program/bank messages", "Keep controller messages", or "Keep other messages" which "adds any other MIDI messages to the score" which perhaps includes system exclusive messages, but I can't tell for sure.
Starting on page 273, the section "MIDI messages" describes how you can put codes beginning with ~ in a staff text item and Sibelius will send corresponding MIDI messages during playback.
* System exclusive messages, described on page 279, begin with ~X. The example (~Xh40,h00,hf7) implies the ~X sends the beginning F0 for you, so you don't need to put that in yourself.
Rodgers i557 i557c i577 i577c Owner's Manual
PDF page 54 (printed page 44) describes how each registration change sends a system exclusive message that contains the status of all the stops and couplers. It explains this system exclusive message could be recorded into a sequencer and then later played back into the organ. But this manual doesn't explain the format of the system exclusive message.
Rodgers i557 i557c i577 i577c Használati Utasítás
This Hungarian manual describes the registration change system exclusive message starting on page 55. The Hungarian text is intelligibly translated by translate.google.com.
* (For comparison, I found the English manual for another model Rodgers organ has an almost identical system exclusive description: Rodgers Insignia 537 Owner's Manual
-- PDF page 44, printed page 42. This helps suggest what the original English description for the i577 probably was.)
My understanding is the format of the full registration change system exclusive message is
F0 Begin System Exclusive
41 Roland/Rodgers SysEx ID
10 Device ID
30 Model ID (30 = Generic organ data) (00 47 = i557/577 data)
12 Data Set Command
01 Subcommand byte
00 Offset byte
... Data bytes (35 bytes)
__ Checksum byte
F7 End Exclusive byte
It's unclear to me if the Model ID should be 30 for "generic organ data" or 00 47 for "i577/577 data" or if either is acceptable.
The Hungarian manual has tables (in English) that describes which bits in the data bytes are enabled for various stops. The English i557/577 manual also says the status of the couplers is contained in the system exclusive message, but the Hungarian manual doesn't seem to say which bits are used for the couplers.
Here are some test codes you can try in Sibelius that I think should send a registration change system exclusive message with all the data bytes zeroed out, which will presumably disable all the stops and couplers. The first one uses the "generic" model ID, the second one uses the i557/577 model ID.