Yes, the plot file would be the quickest way for you create your drawing and be able to mirror your text. I would add a couple more things for you to consider.
1) Create a new board file with a good name i.e. fab_xxxx.brd or drldwg_xxxx.brd, where you can add a new page border and titleblock and then File>Import>IPF to bring the plot file in to the board file. All of the stuff you import (text, lines, shapes, etc) will we written to layers Manufacturing>PenX, PenXX, etc. You can edit and mirror this text. You can plot this or create Gerber if needed.
2) When you import the plot file into the new board file either make your alignment mark (if you want to place it in the exact location as the original board) or work in a very big grid of say 500 mils or 10 mm.
Side note: When you import an IPF you can mirror, rotate and scale the whole plot. After you slelect the plot just RMB (right mouse button) and select mirror, rotate and scale.