Masking of base meshes at load-time is likely not going to happen anytime soon as it would require a major overhaul of the rendering engine and DSF spec.
However, combining of landclass and orthophoto mesh is already supported by the current DSF specification.
The problem is that currently there are no scenery authoring tools available that support this. Since OrbX use Ortho4XP to create their orthophoto scenery they are currently depending on it. Hence, for them it's actually an Ortho4XP limitation.
But since they have a big dev team they should look into writing their own tool. Or better: Contribute to Ortho4XP development - which is open source.