To be fair there is fluff out there for them looting imperial vehicles. There is a good story about them looting a Land Raider
So the fluff would be backed up if you did it as a looted Imperial Knight rather than one they have built themselves.
Basically that's exactly the fluff I would want to use
Looted Imperial Knight, but "harlequinised".
The model itself has some nice, smooth curves. Very eldary-like. Also, the giant skull for the head is absolutely perfect
I will probably add some spirit stones. Definitely painting motley on some parts of the armour. Also I'd like to add some bones, but not sure, if it's enough to paint them, or if I should try to convert them somehow (definitely not able to sculpt them from greenstuff, lol).