420 SEL Timing chain/guides - failure risk
The timing chain rail breakage on the 420s would be described by anyone as a catastrophic failure and, as such, you can't really put a time limit/mileage range around it.
Some have said that frequent oil changes mean that you shouldn't worry; others have said look at the color of the guides. No disrespect meant here whatsoever, but a catastrophic failure is a catastrophic faiure. Another example that has been discussed recently on this list is the single row timing chain in the 3.8 engines. One could change oil every 500 (five hundred) miles in this engine and yet, you are still "rolling the dice" on continued driving with the single-row chain. Same with the problematic guide; a less-than-$5 part in this case . . .
I would venture a guess that the major explanatory factors for guide failures are the number of hot/cool cycles and the amount of in-town driving, because we're talking about a plastic guide here. The only reliable visual in my mind would be hairline stress cracks; I know that several have seen this evidence on guides. Color to me would provide minimal information unless it is near black.
Consequently, back to the original poster's question: "Is the car safe to drive on vacation?" As someone has already pointed out, guide failure is unlikely to occur in this 6-700 mile interval. However, life is a series of risk assessments. You have a known catastrophic failure for your specific engine. How much risk are you willing to assume for this known problem?