well, you need a new Orion model, so just copy and rename the one you have, then copy the table data (use notepad to open ships.tbl and find the Orion entry), rename the POF File entry to the new Orion POF you created, then save.
Then you need to make the new nameplate. Instructions on how are stated above, so I'll skip that. When your nameplate is complete, save it in 256 pcx, as a new file (e.g. CustomOrion1.pcx). You don't want the nameplate to show on the regular Orion, right?
Now you need a POF editor. Either Kazan's PCS or the latest edition of Modelview will do. All you need to do, is to open the custom nameplated Orion POF, replace the current nameplate texture (simple rename thing), with the custom one, save and tadaa!