As the RPW is to be guided by (and subservient to) the Scriptures, shouldn't even a passage like this, however anomalous it may seem, be considered? If you happen to find somebody who adequately answers this passage in relation to RPW via sermon or writing, and does it without making any assumptions not laid out in the text, I would be very happy to read it.
You are appealing to the RPW to allow a conclusion that directly violates the RPW. Something is amiss there. The RPW is, What is not commanded is forbidden. The appeal to a non-commanded feast is fitted to overturn that rule. It effectively says, the RPW cannot be right because Jesus worshipped at a feast which God never appointed.
Scripture is the only rule of faith and life; but Scripture does not rule all things in the same way. See WCF 20.2. There is the normative principle for all of life. The Scripture contains the moral law, which is applicable at all times and in all places. It says, What is not forbidden is permissible as long at it is expedient and edifying. Besides this "moral" rule for all of life, Scripture contains "positive" prescriptions for faith and worship. These positive prescriptions depend entirely on the will of the lawgiver and they are only applicable for the action prescribed. This is the regulative principle, which says, What is not commanded is forbidden, even if it is expedient and edifying. It does not depend on our reason but on the will of God alone. As God Himself is unsearchable and His ways past finding out, so also the Name which we worship is too wonderful for us to adore and appreciate without the disclosure of His own will. His majesty is too high to allow commoners to be so familiar with him as to tread His dignity under foot; as a monarch deigns to associate with his people under the strictest protocols which preserve his dignity, so the Most High graciously permits and invites our approach to Him but in a way that is accordant with His own majesty. One does not look at the general law of the land to see how the monarch is to be approached, but attention must be given to the specific protocols which the royal house has prescribed. In the same way, we do not look to the general rules of Scripture which govern all of life when we have in mind to worship our Sovereign, but we search out the specific form in which our great God and Saviour has prescribed for us to draw near to Him.