Arousal scaling is based on the arousal variable for in game characters, which rises as a character is "allured". I could alter the scaling to also rise over the duration of sex possibly.
For female characters, I'll see if I can attach a fluid system to them. Normally this would be trivial, but almost all of this game's systems have a lot of extra hoops to jump through.
Non-base-game NPCs can have precum and all other settings set by selecting the character in the character mod menu. I plan on supporting built in NPCs, but for now, you can disable them in the main game menu under "manage npcs" (they are highlighted pink). It's worth mentioning the base game has a bug where NPCs of the same name will be disabled just by disabling one of them.
The game uses a decal system for projecting fluid onto surfaces, so I'll need to look into some of the renderer code etc. to see if I can tweak this without requiring another patch file.
Copying appearance data should be easy enough to add.