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The current SCSI Manager 4.3 interface is not guaranteed to be compatible with future Mac OS releases. At this time the SIM architecture is not fully defined and may be subject to change. However, it is possible to write a fully native SIM by passing universal procedure pointers to the XPT layer for the SIM's entry points and by using CallUniversalProc in native code to call the XPT's entry points. This approach is outlined in SIMs for Current Versions of Mac OS. Universal procedure pointers are described in Inside Macintosh: PowerPC System Software, listed in Apple Publications.
It is also possible to reduce the effort required to become compatible with future releases of Mac OS by following the rules set forth for other drivers in Writing Native Drivers Primarily, you should limit communication with Mac OS to the calls documented in Driver Services Library