sooo. .. I think I got it working...
Annoyingly I do not know quite how..
What I did was... try multiple versions of nanoboot..
Then I tried the synoboot.img from this site.. still didn't work..
Then I deleted all vdi files, vmdk's etc. that I tested..
I then booted with nanoboot *4528
and installed version 4528 from xpenology.nl
This combo.. which I've tried before.. seems to work.
Harddisk in Vbox is 50 Gb Sata.