FYI, I emailed the gift planning folks at the church about using DAF for misisons and fast offerings (I specifically asked about the Schwab DAF, but I'm sure the process is the same for other brokerages). This was the response:
Donor advised funds may be used to make fast offering, tithing and missionary contributions. In order to do so:
Instruct Charles Schwab to send the money to the Corporation of the President of The Church of Jesus Christ of Latter-day Saints (TIN 23-7300405). If possible, on the memo or grant purpose line, include your membership record number, ward and stake unit (for tithing/fast offerings); for missionaries, include the ward and stake and name of the missionary for whom the funds are to be used.
If the grant/memo line cannot contain all the information, of if you would prefer, you may notify Treasury Services by email at email@example.com that the distribution is being made. Include your name, Church record number, mailing address, name of DAF administrator (Charles Schwab), exact amount contributed, and allocation of the donation among tithing, fast offerings, and other funds of the Church. This will help ensure that the donation is credited properly. You may also contact Treasury Services at 801-240-2554 if there are questions or additional information is needed.