I cannot comment on the specific tent you're considering, but I did use a UL tent (zPacks Hexamid Twin) for many years in the Sierra Nevada and other locations where it was not possible to stake the shelter down into the ground. The method I used was to find large rocks. I rigged guy lines to the shelter longer than the standard lines to make it possible to make loops around the large rocks. This worked anywhere large rocks could be found, which is most anywhere in the Sierra Nevada. The pitch was sometimes not perfect but I rarely had a problem with the shelter failing to stay upright. One of the first nights I experienced this was at a very windy campsite at Thousand Island Lake. I somehow achieved a nearly perfect pitch and I had almost no experience with the shelter at that point, pictured below. I would recommend practicing before going out on the JMT. Picture of the setup, from my trip in 2013, is below.
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ravi_jmt2013