Cloning system while preserving target identity (e.g. DynDNS)

[Syzygies]

I've just wasted the past hour, taking the term "Ignore" at its word. "Smart Update" is literally a faster erase-then-copy, which means that "Ignore" never means "leave alone what's there".

Let me repeat my application: My MacBook lives in my office, my Mac mini lives in my living room. There are a few key identity issues I'd like to keep separate, between the two machines, but I don't want to waste half my life repeating software installs and updates separately to the two machines. This seems like a dream, stock application of SuperDuper! Clone one machine to the other, but tell SD! for instance to leave alone the directory "/usr/local/dyndns/Accounts/" and the two machines can continue to have separate DynDNS identities.

But no. "Ignore" instead deletes the named directory, which has the effect of instead crippling the target machine.

So how do I tell my script Clone.dset to neither delete the existing contents of a target directory, nor copy the different contents from source?

(Please don't tell me to handle this with before and after shell scripts.)