I've done the day hike to Reds over Mammoth Pass to start an overnight hike with a permit for the High Trail. ?The rangers I spoke to at the time all said that it was perfectly OK to do that day hike to Red's Meadow the day before the permit. ?They also said that I could walk from Red's to Agnew Meadows on the day of my permit, even though the permit for High Trail doesn't start at Red's but at Agnew Meadows. I think that this is probably still allowed but it would be a good idea to double check with the rangers.
I know that this sort of thing would not be permitted in Yosemite National Park where rangers look with skepticism at hikers with full packs of overnight gear who say they are day hiking. People have had a hard time trying to day hike the Tuolumne to Happy Isles section with a full pack before.
ravi_jmt2013