I am interested to know if any of the 18 Smritis, or Grihyasutras, Srauta Sutras etc praise Shiva as the Supreme and endorse Saivite practices like Bhasmadhārana,and Tripundra etc.
The Highlighted line states Rudra to be Eka, Aadya, Purusha, Puraatana and Vishva-srija; all epithets of Para-Brahman.
Now on Bhasmam/ Tripundra :-
Sri Baudhayana explains herein how the Bhasma that is taken from the Agni Kunda as Prasadam, after the Nitya Agnikarya is done, has to be applied. He states that it has to be applied along with a specific mantra and explains the manner of application as well. The Bhasma has to be applied as Tripundra, which is also how it is applied by Brahmins (Bhasma-dhaaranam). It can be either three horizontal lines or a single line for Brahmins; the mode of application differs for various castes like the Kshatriyas, etc.
A bodhayana grihya sutra prayoga work authored by Kanakasabhapati, also refers to the Bodhayana grihya shesha sutra original and says ‘there is a wide variation in the bhasma raksha Dharana vidhi and one has to follow his family practice.'