I take it you can't just use preamp outputs of an aftermarket HU and make the connections between them and the harness?

No.
HK amp has speaker level connections from HU. It also has communications which aftermarket HU does not do.!

Some info here:
Ians GBMINI website Running Harmon Kardon with no factory head unit (http://www.gbmini.net/wp/2004/04/running_harmon_kardon_with_no_factory_head_unit/)

Other info on my website too - search there for "HKenabler"

anything can be done.. it depends on how much money you want to spend to do it.