It can be as simple as adding a line to the effects section of the aircraft xdp file. If the user has AnKor's shaders installed and have the appropriate lines added to their effects.xml then it will work automatically. If they don't have that then the aircraft just displays the static reticle that you would normally model it with.
The thing for the developer is to model the proper gunsight, and select the appropriate reticle to use with it. Then it's a matter of setting the x,y,z location for the matching reticle effect so it displays on the glass of the gunsight.