I'm planning to replace my aging HP MicroServer and QNAP NAS seems to be a reasonable replacement hardware-wise (quieter, smaller, lower power use, reasonable performance and price). However I'm not interested in QNAP OS, I want to install my custom OS.
Earlier units had standard BIOS based startup that was customizable via VGA/HDMI so you could install custom OS (I've seen some custom OS references for x09, x59, possibly x69).
However there is no firmware recovery guide (yet) for TS-x51/x53 and I couldn't find any images about boot progress from Google to confirm BIOS presence. My concern is that some other manufacturers use custom bootloaders even for x86 hardware and newer QNAP's might have gone down the same route.
My target OS would be Windows Server 2012 R2, VMWare ESXi (Bay Trail-D seems to be capable of 16GB RAM despite claims of only 8GB by Intel) or Debian. I'd probably replace DOM with larger one or use off-the-shelf USB stick (DOM seems to use standard 9pin USB header) and Install OS on it. If that turned out to be problematic, I'd probably use it as rootfs (or Windows bootloader) and do actual boot from HDDs. I do work as senior system administrator so hacks and complicated workarounds do not frighten me

Just to prevent offtopic talk, I'm not interested in running my OS in VM. I'm not interested in (semi)custom ITX hardware (probably more expensive, not as compact, probably higher power use etc...).
I do not expect it to be supported by QNAP (and I would expect my warranty void after such mofification), I'd just like the information prior to purchase.
1. Do TS-x51/x53 use industry standard BIOS to start up?
2. If present, is BIOS accessible via HDMI and USB?
3. If present, is BIOS UEFI aware (2TB+ drives, GPT boot...)?
4. If accessible, is BIOS customizeable enough to boot custom OS installer.