First, in the case of just a 10% (+5% to SteemPlus) beneficiary, the total difference in post payout isn't going to be that huge so maybe this is just making a mountain out of a molehill. There isn't anything wrong with what you have suggested.

The beneficiary bit is coded in, and that distribution happens before the rewards ever reach the post author. E.g. if you set a 50% beneficiary on a post that paid out $12, $3 goes to curators (assuming 25%) and of the remaining $9, 50% would go to the beneficiary in the form of SP... so $4.50 USD worth of SP at current prices. That would leave $4.50 value for the author, which would be split in half per normal, $2.25 USD value of SP and 2.25 SBD. That reduced amount of SP and SBD is all the author would see in their wallet when it came time to claim rewards.

But there isn't any reason a front end couldn't distribute rewards to multiple people at the stage of claiming rewards. So for instance if it were an Open Mic front end, when you claim rewards it could give an option of something like this: Claim 100% of your rewards for yourself; Tip 10% to Open Mic and claim 90%; Tip 25% to Open Mic and claim 75%; etc. This would basically just be a transfer function done behind the scenes - best way would probably be to prompt for active key permission if one of the "tip" options is chosen (vs. storing active key permission which is problematic).

Cheers