I finally had some time to take a look at flash dump and to compare it with "untouched" UEFI image.
Best guess- NVRAM was unreadable for the UEFI.
It seems that NVRAM is stored within the UEFI starting at 0x10358- untouched 314 image on the left, backup on the right:
I also guess that default setup values are stored just before NVRAM, starting (possibly) at 0x10060
However data dumped form the flash chip differs from the default image:
- several bytes also in (possibly) default NVRAM data area have different values (eg. 0x10064, 0x1007B and some other bytes after that)
- starting at 0x1010F data seems to be almost the same in both images but written with a growing offset- at the beginning it's just one byte but later (0x10358) offset grows to 10 bytes
(314 image on the left side , dump on the right)
But this is exactly how 227 image looks like (as before- original on the left side, dump on the right):
Asus WinFlash utility moved NVRAM data (and default setup values) to the place compatible with provided UEFI image and after the reboot UEFI's built-in flasher was responsible for image update. But in my case it refused to work (although I forced WinFlash not to check image build date) and I'm not surprised that T100 didn't want to boot without access to proper NVRAM data.
If only there was a good old "clear cmos" jumper somewhere......