I am talking about using the EGPWS terrain function which paints a nice picture of the terrain around you and its altitude relative to you.
Didn't work too well, for instance, at Mt Erebus ? OK, I'm being a tad facetious. However, the EGPWS terrain databases are not worldwide and, I suspect, not of sufficient accuracy to support OEI planning. Better than nothing, certainly.
I'm not aware of us having ever done a charter to an airfield for which we were not able to obtain the relevant obstacle and terrain data
While satellite data makes a first cut much easier these days, I wouldn't discount your folk making the usual assumptions for one-off charters .. getting adequate terrain data can still entail considerable work and time. For many locations, there exists plenty of data .. can be a pain tracking it down, though, in a short timeframe.
Might be illuminating were you to have a discussion over coffee with your ops eng folks.
GF's observation re HUD is useful although (and I have never operated HUD) it probably is not in the class of an adequate preplanned escape.