This would be similar to the terrain issue in the Contiguous 48 states - East to West. I can't recall the exact position, but there is a point - due to the Rocky Mountains - where "Drift Down / Depressurization" requires a 180 turn, and another point where continue forward is the only option.
After many thousands of hours over (and among) the Rocky Mountains, I've never heard of, nor seen such a point. It doesn't exist. If one is at at low altitude in a performance-limited aircraft, one respects drainages...water flows down hill, and so do we. I've spent a lot of time at very low altitude in the mountains, and religiously adhere to this rule...because mountains can typically outclimb airplanes.
If one is at a higher altitude, one looks at the airports available if this is required, and goes to one of them. If one is close to the point of departure, one may turn back, and if one is in a better spot to continue to one's destination, one goes there.
The rocky mountains aren't all that high to begin with from a transport category perspective, as the highest point in the lower 48 states is only about 15,000' high. This is very high in for a non-transport category aircraft, but otherwise, there are generally plenty of options, and there is no point of no return when crossing the rocky's...and in fact, there are a number of places to go while one is over them.
The example I used - India / Singapore - involves a 180 turn back to a specific waypoint, then a low level airway around the terrain to a diversion alternate (Note: it's an ETOPS route). If I had the route manual in front of me, I could provide the points for better clarity, but I don't have it handy.
You're assuming this is necessary at all. One need not consider only terrain. Circumstances may be that, owing to terrain, distances, remoteness, fuel reserves, etc...one may not be able to descend. This is frequently a concern on an oceanic crossing...where can we go, and how low can we go and still have fuel to reach an alternate? While crossing high terrain, one may be confined to remaining at a higher altitude until past terrain, then descending at the MEA will allow, to reach a suitable destination.
This is one reason to always have a chart available. Engine fails here, we go there. Engine fails there, we go here. At any given time.
On oceanic trips we have equal time points and alternates applicable there-to, sometimes multiple ETP's between multiple alternates, which we know in advance and can recalculate given changes in winds aloft or routing. However, over shorter distances such as as higher terrain under discussion, we don't calculate that, and have no need to calculate that. If we need to remain at altitude for a given period, there's oxygen on board to do that, and we will, until the MSA or MEA or MOCA will allow us lower. In the meanwhile we'll be looking for a place to go, and once that is determined, we'll go there. Simple.