I am a little late to the party on this one, but I'll reply back in hopes that it helps someone out. Yes, you can field install this option, the steps outlined in the manual for the 5334B work just fine.
It's been a while, and I'll need to dig through some old project files for the details. What I do remember is that you need to unsolder and move C8 to C100. I also built a small board to mount the ocxo, provide 12v and add support for electronic frequency control. I then took a panel mount 10-turn potentiometer and stuck it through the back so the unit can be calibrated without opening the case. Finishing touch is a small panel mount LED on the front panel at the top right which indicates that the oven is still warming up and measurements should not be trusted.
If you are still looking to do this mod, let me know. I'll post the project files.