Can a 6 volt relay coil bear more than 6 volt?
A '6v' relay is guaranteed to close at 6v, and to tolerate 6v continuously applied at an ambient temperature within its specification.

If you apply more than 6v to a 6v relay, it will get hotter than intended. 

For a modest overload, like 8v, the only problem will be excess temperature rise, and that takes time to occur. A few seconds is unlikely to overheat the coil.

You have several unknowns. 

1) How hot can the windings get before damage occurs, so insulation smoking on the wires, or plastic assemblies deforming as they get to softening temperatures?
2) How long will it take to get that hot?
3) Will they ever get that hot with 8v and your maximum ambient temperature?

Obviously the relay manufacturer is not going to warrant any of these, so it's up to you to find out by experiment. If you have two identical relays, then you could test one to destruction, and use the other. If you have only one, are you feeling lucky, punk?