1)To me (and I believe for most viewers that agree with that), spitting or drooling spit strings on girls face or inside her mouth or on her pussy and ass gape during a porn scene is a strong expression that you want her very much. An action of intense sexual love and a some short of strong will that you want to cum on her. Actually spitting and sloppy kissing is a form of an introductory way to say that you want to give her and spat her with your cum. It must become a common knowledge, that a true hardcore lover/ participant of a genuine sexual action, that is males and/or t-girls and/or females that dominate a willingly submissive female should have a natural increase of spit production inside their mouths which will become watery even just at the sight of their sexy submissive model let alone when they start to abuse her. Immediately after that, a strong desire to throw it all over her and mostly to her sensitive parts(face,mouth,pussy,ass). Making her look messy like that will amplify their desire and production of spit even more like being a self-increased cycle. The more (messier) the merrier.
The idea(s) of:
a. having a warming up scene with a sexy female model getting groped incessantly and getting messy with french sloppy kisses and spit inside her mouth by a bunch of perv guys(and maybe by t-girls and other females) one after another without carrying to wipe it at any time during the scene and making it happen on and off during entire scene,
b.throwing spit strings (even by more than one guy at a time)in any of her holes(mouth,pussy,ass) and especially whenever a guy is about to shove his cock inside it and
c.throwing spits at random points- but quite often- on her face and especially while she is fucked,
Should be top-hot priorities in any hardcore rough scene that respects itself.
2)You should pursuit also, for the submissive model, to produce much much much of her own spit.Of course, ideal way of achieving that is by forcing big cocks deep down her throat with her mouth wide open and make sure to keep her in a position that her own produced spit should drool/run down her chin,tits, pussy and thighs. Some puke should be OK occasionally. Some models are able to make that cocks really messy with her own spit while they fuck the bottom of her throat. That messy cock should go immediately inside her ass. No need for artificial lubricant at that point!

PS1. In my opinion if pissing takes place should be used only at the beginning of the scene and order the model to swallow it (all if possible) and maybe some during penetration. Pissing should not clean out any spit and cum stains at any point. Spit and cum stains embellish the view of the model. So they should be sustained and increased for as long as possible on the flesh of the model and especially on her face and on her holes.
PS2. A heavily stained with spit and cum model while she is still being used and abused and creampied and cum stained again and again is a really unique spectacle that should be considered to apply in every scene