Is upanishadic concept Brahman (neuter gendered word ब्रह्मं meaning absolute reality/all encompassing being) mentioned anywhere in Valmiki Ramayana? I only remember reading about brahma, a grandfather of all etc (ब्रह्मः masculine gendered word)
Yes, the concept of the Brahman is mentioned in Valmiki Ramayana:
एते सेन्द्रास्त्रयो लोकाः सिद्धाश्च परमर्षयः |
अभिगम्य महात्मानमर्चन्ति पुरुषोत्तमम् || ६-११९-३१
एतत्तदुक्तमव्यक्तमक्षरं ब्रह्मनिर्मितम् |
देवानां हृदयं सौम्य गुह्यं रामः परन्तपः || ६-११९-३२
These three worlds, including Indra Siddhas and great sages pay homage to the great souled Rama as a Supreme person, on approaching him. O great one! O the destroyer of adversaries! Rama has been spoken of as the unmanifest and the imperishable Brahman (the absolute), established by the Vedas, the heart of Gods and the secret of all Gods." (from Sarga 119 of YUDDA KANDA)
भवान्नारायणो देवः श्रीमांश्चक्रायुधः प्रभुः || ६-११७-१३
एकशृङ्गो वराहस्त्वं भूतभव्यसपत्नजित् |
अक्षरं ब्रह्म सत्यं च मध्ये चान्ते च राघव || ६-११७-१४
लोकानां त्वं परो धर्मो विष्वक्सेनश्चतुर्भजः |
You are the Lord Narayana himself the glorious god, who wields the discus. You are the Divine Boar with a single tusk, the conqueror of your past and future enemies. You are Brahman, the imperishable, the Truth abiding in the middle as well as at the end of the universe. You are the supreme righteousness of people, whose powers go everywhere. You are the four armed. (from Sarga 117 of YUDDA KANDA)
It has been written in many parts of Valmiki Ramayan.
एतत्तदुक्तमव्यक्तमक्षरं ब्रह्मनिर्मितम् | देवानां हृदयं सौम्य गुह्यं रामः परन्तपः ||
Valmiki Ramayan 6.119.32
O great one! O the destroyer of adversaries! Rama has been spoken of as the unmanifest and the imperishable Brahman (the absolute), established by the Vedas, the heart of Gods and the secret of all Gods.
Now the original version of Valmiki Ramayan tends to have this shlok but it has entirely ommited from ramayan The shlok goes as this:-
परं ब्रह्म परं तत्वं परं ज्ञानं परं तपः। परं बीजं परं श्रेत्रं परं कारण कारणम्।।
Valmiki Ramayan quoted in ram navratn saar sangrah, Tulsi satsai tika, Manas piyush and ram paratvam
Shri Ram is parambrahman, param tatva, greatest knowledge, supreme penance, seed of all bijja (om, hreem, shreem, kleem, kreem, gan, dam etc ), greatest place and greatest reason behind all of reasons.
Hope my answer helps