Great question. We are currently working on a direct way of querying the height of terrain given lat/long.
However, given your use case, a decal component might be well suited for your use case of drawing on top of the terrain.
If you do need to get the height in the meantime, you could try doing a line trace to the world terrain object and use the Z value. The only issue might be if the fully refined tile is not loaded in when doing the trace, in which case the resulting terrain altitude will either end up being an overestimate or underestimate.
You could work around this by a) doing a new line trace when the camera gets close to the area and more detailed terrain gets loaded in, and adjusting the height of your Unreal object, or b) Temporarily setting the Maximum Screen Space Error in the tileset to a small number, and using the fully-loaded terrain to do the line trace (for example, using a small max SSE at editor time, then once the heights are determined, the value can be set back to normal).