So many great answers, but I couldn't read through it all, I was getting too upset.
I keep thinking of this awesome bib, it says "SPIT HAPPENS" in bold colours. It happens. Children don't yet understand how harsh it is to spit in someones face, they haven't seen movies, or had it happen to them. Like the first time a child pees on the floor on purpose, you get annoyed, explain, have a huge hug, and they wont do it again (hopefully), but if you yelled at them, forced them to clean it while crying (or something else horrible like this) they might continue out of fear, anger, confusion, etc.
Spit happens. It's okay to be mad, but putting a child in harms way over something that could be used for leaning is scary. I just think in heated moments like this while the DH is holding his DD down to force it into her mouth, maybe sitting on the toilet, she could wiggle and hit her head or something.
Not that the soap isn't bad enough, because trust me i KNOW it is, but if he is that mad, he could lose control.
If this was my DP, I would have him put DD to sleep (if this works in your family), make sure they got lots of time together in the bath, playing, reading etc before bed (I'd be watching the WHOLE thing), have loads of snuggles and let the appologies come out naturally. When DC was alseep, I'd have a huge talk about what happened prior to the spitting that made her lash out, and how he could have handled it, reminding him, no one is perfect, and everyone makes mistake, but we CHOOSE which mistake we make.
If he has a history of violent bouts (has he ever lost it on you??) I'd get him to agree to seeing a family therapist, or on his own, depeding on his past.
I would also mention, that walking away is a MUCH safer way to handle the situation, calm down for a minute. This is a very 'real' concequence for a child too, they don't like it, they don't get a reaction, they will probably be upset and get that spitting isn't okay without words and abuse.
Okay, I'm very upset. I hope that came out alright. Hugs to everyone.
And, soaping a childs mouth is very very much child abuse.