The problem got worse and worse. Just last week I had to insert and pull out the empeg probably 30-40 times until it would boot.
In a desperate attempt this weekend, I pulled out the old soldering iron and re-heated and/or resoldered every single contact on the memory chips - and so far it's been booting like new!
So, just for the record/archive: the empeg worked fine for months after the piggy-back memory upgrade. Then random sigkill-errors appeared during boot. The cure: re-solder the memory pins. All of them.

I just hope the problem doesn't come back.

Thanks for all your help, Rob!