Our initial response would be that the signature panel needs to go on last, the main concern being that the signature panel is raised off the surface of the card, which may interfere with the print quality.
However, certain manufacturers have suggested that this is not the case. They have reported that there will not be any damage to the print head, however some adjustments may need to be made, or the quality will not look as good.
If you are planning on preprinting a signature panel, and then printing over it with YMCKO, the adjustments they recommend making for the YMCK sections of the ribbon would be to have a knockout in the design. This way the printer will not print on that area.
The main issue would be the O, or overlay section. If the overlay is not disabled in the signature panel area, then the overlay will not apply to the signature panel, but will leave a very faint, odd design around the panel where the overlay did not apply correctly. To prevent this, it would need to be specified where not to apply the overlay.
Overall, we would not suggest printing the signature panel first, but if required, these are some things to take into consideration.