Should not be too difficult. I haven't tried the "Bush People" mod, but I have disabled the default underwear on player characters in my game.
It is basically an "attachment" defined in PlayerPawnTest blueprint (2 in PlayerPawnTest_Female and 1 in PlayerPawnTest_Male). So the first step is to remove those. Then you have to either disable or set to default clothing masks (Human_Female_Body_Colorized_BASEMIC / Human_Female_Body_Colorized_BASEMIC) for shirt and/or pants. These textures are referenced from Human_Male_TPV_Skeleton / Human_Female_TPV_Skeleton, so you have to also override these