I personally don't like it when a custom livery appears on an AI service. I think it more depends on what train or wagon the livery is on. For example I like custom liveries substituting on Sand Patch Grade trains but not so much on Clinchfield or LGV. I think every individual livery you make in livery editor should have an option to enable or disable AI substitution for that specific livery. That way I can be like, "I want this red and black boxcar to be seen on AI trains, but I don't think I want this passenger carriage to have a green livery every time I pass one" Maybe this can be a feature with Livery Designer 2.0
I highly agree with this as I make liveries all the time. I don't want all AI trains to be custom liveries.
And it would be nice to add to this an option where you can choose how often a custom livery spawns in the game for those of us who like seeing our creations on AI trains, but may not like how often they spawn on a train.
Perhaps an option to limit the number of instances in any game, so if you do a specific numbered livery, you can set it to only appear once, whereas if you do something more generic, you could set it to more/default/random so it would just get mixed in with the other liveries.