What should I plan for on a sea-to-summit ride on Mauna Kea, Hawaii?
I think this long post by Martin Hoff also indicated by jurgemaister in a comment tells you everything you need. You can find some more info in the comments on individual attempts people did of the segment on strava. The main takeaways I got from the article are:


You need two bikes (a road bike and a mountainbike) or a dirt-road suitable bike.
You optimally have a support vehicle driving along with you for food/drinks/extra clothing/emergencies/the way down/nice pictures/the second bike.
If support isn't an option you need to pack for a really long hard day in both winter and summer climates. Even if you are very fit. Use panniers mounted on your bike, a backpack carrying enough stuff will probably break you.
Mauna Keau tops out at over 4K above sea level. How fit will you be up there? How used are you to high altitudes? Altitude sickness might be an issue, strenuous activity will certainly be very hard.
You need low gearing for the steepest part. Install a mountainbike crank-set.
If you take one bike you probably want knobbly tires on it.