I have pointed this out time and time again, most recently earlier in this thread
Based on the privacy that the Jobs family coveted, they would indeed seem to be the most likely American billionaire family to be anonymous donors.
By donating anonymously, especially at a top tier, you aren't constantly barraged by requests from development officers from whatever random organization. It's a more efficient use of the donor's time.
If the Jobs Trust donated publicly to SFMOMA, they would be fielding calls from every single art museum within the state of California, and probably most of the big ones all over the United States.
Remember that Laurene Powell is heavily involved in numerous charitable organizations, yet remains almost completely silent. Generally speaking, someone who is at the board level of a charitable organization is also financially involved to a degree.
If you look at those program guides from the symphony/opera/ballet/whatever and compare the board/governors/etc. with the donor list, you will find almost all of them as donors. Yet there will be a few mystery anonymous donors, even at the highest levels.
If Laurene Powell Jobs is sitting on the board of some charity, it is likely that there is at least one anonymous contributor on the top tier donor roster and it is also very likely that at the Jobs Trust would be one of those anonymous donors. If she is spending time with an organization, it is likely she is spending money with the organization as well. If she really wanted to, she could just cut a check and forget about it. By giving her time, she is taking away from others (family, friends, herself).
This is different than putting a twenty dollar bill into a Salvation Army kettle during the Christmas holidays.
Let's say the Jobs Trust donated anonymously to SFMOMA, but somehow it got leaked amongst the VC, museum directors, country clobbers, and old blue bloods. Let's say this information made it to the development department at the Fine Arts Museums of San Francisco. Do you think the development director would approach the Jobs Trust for a donation? And what does the SFMOMA development director do? Ask for more?
There are a lot of power politics in play when a top tier anonymous donor silently appears on your doorstep. By staying anonymous in their charitable activities, a couple like Steve and Laurene would exert maximum control over their gifting. They wouldn't have to field ten calls and reply, "Sorry..." and gently explain why they're not donating to Organization X, Y or Z.
Concerning Stanford Hospital & Clinics, it's quite possible and even probable that a major individual (non-corporate) donor will step up. After all the Corporate Partner Program will only cover a fraction of the projected costs. Who would be the most likely person to donate anonymously as an individual to Stanford Hospital & Clinics?