In Tantra, is there any mention of "cows being sacred"? Also, does Kaulācāra Tantra explicitly talk about it?
Yes obviously. Like all other Hindu scriptures they also hold the same opinion about cows.
Trinam vapya-vidhAnena chedayenna kadAchana |
VidhinA gAm dvijam vApi hatvA pApairna lipyate ||
If it is not prescribed in scriptures, then never even tear a piece of grass. But if prescribed in scriptures, one does not commit any sin by killing even Brahmins and cows.
KulArnava Tantram 2.137
This is showing that killing cows is a grave sin and hence the sacredness of cows follows.
This verse is talking about the fact that if animal sacrifice is prescribed in the scriptures then carrying it out is not a sin because it is allowed in the scriptures. Killing in sacrifice is non-killing as say so many other scriptures.
Yet another verse from the same Tantra:
Go-brAhmana-vadham kritvA yat pApam sama-vApnuyAt |
Tat-pApam sama-vApnoti gurvagrehanrita-bhAshanAt ||
If a disciple utters a lie in front of the Guru then he commits a sin that is equal to the sin of killing cow and brahmin.
Lying itself is an act of sin but when done in front of the Guru then the sin assumes a bigger proportion and which then is equal to the sin of killing a cow. So, from this verse too sacredness of cows follows.