It's not that - Synergy is on the most recent modding codebase available. It's just that there's A: no way of incorporating additional code (the new effects etc) from a mod mounted in Synergy and B: Because the Cinematic mod uses such huge, huge files, it needs a hacked up version of HL2 to support it. If you try and use them in any other mod, it will probably crash.
There's nothing to stop you making a few model or texture replacements of your choice however - just install them into either the Synergy folder (if you only want them in Syn) or the HL2 folder as you would normally.
Well that was the point of my question actually, whether the content would load under synergy. The light version of CM says 32bit works so I wasn't sure if cinematic mod still used hacked binaries in that case. It might be that the post I was thinking of was on the CM discussion forum though and related to CM being out of date code, or I was just misunderstanding.
It's been a few years since I had time to keep track of Hammer (&UDK etc) so I can see where I might be mistaken on that. In any case I'm not sure I have time to make a port of CM to synergy myself

but the wife did start exploring HL2 with me yesterday and I'm going to get her down here in a minute to do some more...so thanks for the responses! I'll just play CM on my own in parallel if I want to check out their work...