HEX editing is supposedly one of the capabilities in S_K's PEInfo:
http://s-k.al.ru/wincepdaru.html
I've had a look in the past, but not being much of a HEX editor I can't say for sure if it really works, or what the limits might be. Seemed busted to me, but perhaps not.
I saw another out there somewhere. Forget the name. It was freeware as well, if I recall right, but nothing of the name or location stayed in my memory.