Our figure for diversion en route your looking at one hour single eng cruise speed defined as 395kts(737-800/900). This is the non etops figure obviously.
From personal experiance you would have no prob maintaining this as a single eng cruise speed even on the under powered -900 but you won't be able to maintain cruise fl if you have anyone on board we had to drop to fl160. But performance wise no probs at all. You should have tables you can use for max cont. and fuel planning that are worked out on standard drift downs if you want to get an idea of worst case they have been thinking of. But if you getting involved in these when the "blank" is hitting the fan No. 1 your not using your fmc enough and No. 2 you a better man than me.
But as you said if it doesn't happen in cruise then that doesn't really work also you have to remember these are still air figures. Now if you are in the climb, are you gonna continue to climb??? i should think not if you can clear terain and are going to divert within afore mentioned dist. so basically you looking at leveling off at a suitable speed any sugestions??? I'd say you could easily achieve standard 250kts or if you wanna make it even easier you could go for 220kts (clean but already slowed to start an approach). These are poss figures you could use for youself and for preplanning on the ground. But this is by no means standard and therfore if you are using it i suggest you get your buddy on the flight deck with you in the picture. Remember of course losing an eng is not the end of the world level off settle yourself and dont rush it set up and hold somewhere if you need the time to prepare in europe you should have a prob with a diversion within and hours flying.