yes, I don't need its decoding capability and I don't think its syntax is pretty but if I ever decide to write my own library I'll probably wrap xed in some C++ sugar.
I'm not sure I understand your requirements but might DynASM be useful? It's one component of the JIT library behind LuaJIT but many people use it for run-time code generation completely outside a JIT setting.
DynASM looks cool but its preprocessing step and fancy C integration definitely place it outside the description "Just an instruction encoder with a moderately pretty syntax."
I guess it's not fair to say that XED and DynASM are "much more complicated than the problems they're trying to solve." They are much more complicated than the problem I'm trying to solve. But I am surprised that there is no minimal X86 encoder with nice C++ syntax out there.
http://luajit.org/dynasm.html