http://sn.1w6.org/file/mk-20140918T035050-xbzyd8j.html
oh great - I found a thread on the asustor site that not only confirms what I found (that the fstab gets replaced every boot) but shows how to run your own script at startup (so you can mount drives 'manually' without changing fstab - which is exactly what I was planning to do); good to know I'm on the right track :)