Can a rollover IRA beneficiary designation be drafted using "separate account" language leaving a percentage to a trust qualifying as a designated beneficiary (i.e., 75% to the trust) and the remainder to a charity and have the minimum distributions to the owner be calculated on the basis of the owner's and designated beneficiaries as to the 75% and on the owner's sole life expectancy as to the 25% going to the charity?

I think you need to establish separate IRAs prior to the required beginning date. A separate account approach doesn't change the fact that a beneficiary of the IRA is the charity because the separate account is not established at the time of the beneficiary designation but at the time of death.