@StarsInTheSky:
I don't know if I misunderstood you, but HIEW "takes asm code and writes apropriate hex". When you are editing in disassebler mode, you can press F2 to get a window where you can input ASM code and HIEW will translate it to the correct opcodes. There is no "plugin" needed. I don't even know how someone should implement a "plugin" other than patching HIEW directly.