Orthomosaic / Handling white areas & Tool recommendation for ortho retouche

[klemspect]

Dear Forum and especially @Alexey,

i think these are two simple question for you:

• Is there any option to have the same interpolation (or call it "filling algorithmin" based on surrounding vertex colors) with orthomosaics that is used for 2.5D height-field meshes?
  This is exactly what i need but would have to add this manually in photoshop postprocessing.
• What tool do you recommend for retouche of large orthomosaics (+1k Mpx)? GIMP crashes on my system (although 128GB RAM), does Photoshop work with such large files? Is there any other tool especially for orthos?

Background:
I am generating orthomosaic for paths within forests. With terrestrial images (camera on a pole of ~4m height, facing down as vertical as possible), i generate models. Eventually i merge all the paths as they do not align in a single batch (seasonality, light). Of course, there are lots of white areas in the ortho, that i would love to have filled exactly as it happens with 2.5D meshes. I have just recently started using metahsape and am not yet that familiar enough, so i ask this question here.

thanks
klem