Software

Current Linux implementation (Netatalk) of Apple Filing Protocol does support all the good stuff properly. Mainly:

• DHX2 (Diffie-Hellman Key Exchange 2) -based authentication: the old auth was plain-text, this is a huge improvement, all modern OS Xs have this as mandatory
  
• AFP Lock Stealing, + couple of other features: See Apple's requirements for Time Machine server
  

In my case, there is a HFS+ sparse file on an EXT4-partition. Setting this up with Time Machine is a breeze, however TM will "think" a while before mounting the sparse file and actually running the backup. There are a couple of user testimonials that occasionally (too often) TM will freeze and fail to do a backup. Also when the failure occurs, TM will fail to continue operation and a full backup media reset will be required.

The source of my information is QNAP wiki, which describes the process on old(ish) Mac OS X and Netatalk. I got my backup running based on that information. Also, if there are earlier reported failures to backup, I'll report back here.