Jain temples where the Mulnayak (principal deity) is a Siddha Bhagwan exist, and many famous Siddha Kshetras are known for this. Here are some well-known examples that you can explore further:

• Shri Digambar Jain Siddha Kshetra Shatrunjayaji, Gujarat (Palitana hill complex) – Siddha Kshetra tradition; the site is revered as a Siddha Kshetra with multiple Mulnayak idols in various temples here.
• Shri Digambar Jain Siddha Kshetra Gajpantha-Mhasrul, Maharashtra – A Siddha Kshetra with a sambandh to many Munis who attained siddhi; the place is treated as a Siddha Kshetra. Mulnayak idol information is listed in the Siddha Kshetra profile.
• Shri Kalikund Parshwanath Digambar Jain Siddha(Atishay) Kshetra, Kundal, Maharashtra – A Siddha Kshetra with Parshwanath as a central Mulnayak in some configurations, known for its Siddha Kshetra status.

Notes

• Mulnayak in Siddha Kshetras is often described in tradition as a Siddha Bhagwan (liberated soul) or as one of the Siddha lineage honored at that site. Traditions can vary between Digambar and Shwetambar communities, and temple-specific Mulnayak keepsake may differ by sect and local practice.
• If you want more precise Mulnayak identifications for a given Siddha Kshetra, I can look up the exact Mulnayak idol for that temple and summarize it clearly.

You can read more about these Siddha Kshetra temples and their context here:

• Shri Digambar Jain Siddha Kshetra Gajpantha-Mhasrul, Maharashtra
• Shri Kalikund Parshwanath Digambar Jain Siddha(Atishay) Kshetra, Kundal, Maharashtra
• Shri Digambar Jain Siddha Kshetra Shatrunjayaji, Gujarat