It is very interesting to know that Adi Shankaracharya would have written Bhashya on Yoga Sutras of Patanjali. The idea of Adi Shankaracharya's view on Patanjali Yoga Sutras often comes in my mind and the only thing known/available is that in his commentary on BrahmaSutras, Adi Shankaracharya refuted the Yoga School just by a single paragraph. (Actually BrahmaSutras debates with Samkhya considerably and when it comes to Yoga, it says since Samkhya is refuted, Yoga is also). This was one of the reason for asking the question: Has Swami Vivekananda discussed regarding the refutation of Yoga Darshana in Vedanta Sutras?. The answer gives us an idea that the practice of Yoga and philosophy of Yoga can be different things and practice of Yoga would be accepted by Vedanta.
Coming back to the point of question, Yes!, I've just found a scanned book Patanjala-Yogasutra-Bhasya Vivaranam of Sankara-Bhagavatpada on Internet Archive. It's tagged under opensource collection.
Here is the detail of book:
Rama Sastri & Krishnamurthi Sastri, S. R. (Eds.) Pātn̄jala[sic]-yogasūtra-bhāṣya Vivaraṇam of Śaṅkara-Bhagavatpāda Critically Edited with Introduction (Madras: Government Oriental Manuscripts Library, 1952), Madras Govt. Oriental Series no. 94.
Here is a sample image of the book:
You can get it in PDF format 64MB
Note: The authenticity of the work is also discussed in preface.
As mentioned in the question the english translation of Adi Shankara's sub-commentary (Vivarana) on Vyasa's Patanjali Yoga sutras authored by Trevor Leggett is also available in English
on his Adhyatma Yoga Trust Website which we can read over there.