This means that not the whole Hotsync stuff uses Carbon API - which is ok - and CFM executable format, the binary format used prior MacOS X, and not mach-o as in MacOS X.
Commercial solutions do exist (Missing Sync), and Tiger is expected to provide open API for iSync, which may do the job. But indeed it is possible to write a HotSync conduit with MacOS 10.3.x

A conduit is no more than a plug-in, i.e. a piece of code that is linked with
 • Hotsync Libraries, CFM libraries providings several helpers
 • with the calling application

Beeing called by a CFM binary is no probleme, thanks to bundles.

Calling functions of a CFM libraries is although straight forward using bundles. But this will not work for hotsync functions (except very simple ones), because these functions will uses symbols from the calling application (the hotsync manager). E.g. you don't want to load a libraries, but to access symbols already mapped. (at least this was I understood).
One therefore need to write custom wrapper to call HotSync functions as LogAddEntry, SyncRegisterConduit etc.
Example of such functions can be found at cwMachOCFM by Uli Kusterer. Most of the wrapper in cfmglue.cpp uses these functions.
Sources of the MoCxConduit (the conduit part of MacMoCroix) are available in the download area, under the Gnu Public Licence. Note that right of use of the MachOCFM code itself is not cleared (code published but no explicit licence found).

Note that in cfmglue.cpp, only few HotSync functions are mapped - only the one needed by this conduit. Macro framework should make it easy to add other functions. However, some functions, like UmSetUserTempSyncPreferences apparently does not work. This has not been investigated.
Although conduit provides a direct conduit function, the overall idea was to have the conduit behave as a proxy, which start -if needed- an application (possibly in Cocoa), the same way iSync does. This is what is achieved with MacMoCroix, but only for very few functions.

Note: compilation on Intel Mac (PowerPC target) seems to cause problem.