Thanks to the kindness of a squadmember, I have a customized unofficial .bmp P-51 skin that should work for tooling around offline.
However, since the new skins are saved in .res format, I'm not sure how to set things up to enable the custom one for offline use. Putting the file in the skins folder didnt work (which made sense after I thought about it), and creating a subfolder identified as P51D25 didnt let it show up either.
Any ideas?