For anyone interested, I somehow solved this issue by re-applying the DRAC firmware (same version, 1.70 build 21) and then recycling two serial communication settings in the bios (=switching two settings back and forth, in the end the config is the same). I don't think the bios settings had anything to do with this, otherwise reverting them to my original values would have caused the problem to reappear. I suspect there was some kind of DRAC firmware corruption, but at the same time I'd expect much more problems in that case... I'm 100% sure though that re-applying the DRAC firmware and recycling the two bios settings were the only changes I made between the problematic and the working keyboard.
Based on DTK v3.5 user's guide, I've created a Windows PE 3.1 bootable media with DTK 3.5.1 A00 included, and have weird keyboard issues. When used with iDRAC6's virtual console, the "u", "l" and "p" keys do not work (no output) unless they're used with "Alt" or shift keys. Outside the WinPE 3.1 bootable media (i.e. in the BIOS), no problem whatsoever, the keyboard works fine through iDRAC6's virtual console. And also no keyboard problem with the same media when used locally on the server (=no iDRAC console). Btw, I'm using the latest available versions of WAIK (2.0 + supplement for W7 SP1), DTK (3.5.1 A00) and iDRAC6 (1.70 A02).   I guess it's a bug between Windows PE 3.1 and iDRAC6's virtual console? Or am I missing something?
Weird keyboard issue with WinPE 3.1 and iDRAC6 console