If anyone is interested especially for expedition areas. I have just completed a methodology to import SRTM DEM data into Survex. The SRTM DEM provides elevations on a 90m spacing horizontal grid for the world based upon the WGS84 Horizontal Datum and WGS84 Ellipsoid.

The method involves downloading the appropriate SRTM v4 5degree tile, subsetting this to the area of interest and converting to UTM co-ordinates using a shareware package 3DEM which then exports the DEM subset as .txt and .hdr files. The utility I have written then reads the .txt and .hdr files to generate a .svx file which contains all the elevation values and the UTM co-ordinates as fix fields and then creates all the wires between these.

Some limitations of the utility, less than 800000 data points, rectangular area. Could increase the number of data points if needed.
Have tested it with an area covering Mulu with about 50,000 data points - looks nice and it now includes Gunung Mulu.
There might be an issue if your expedition area crosses a UTM zone boundary. 

If you need a terrain model for your area for use in Survex, feel free to contact me with your desired latitude and longitude boundaries.