The only way I can think of to o that is with some kind way to keep the beeer pressurized during the transfer.
How about when you use a 15 gallon for parties you transfer the last five gallons ro whateveris left into your cornelius after theparty?
You'll want to keep the rental keg cool and out of sunlight until you are ready to go.
You could use a bleeder valve or a spunding valve to keep the pressure real steady, or you could set your CO2 tank to say 20psi, hook that to the rental keg, and then tap the pressure relief valve on your target keg every little bitto keep the beer flowing.
You are going to need some way to get from Sanke (commercial beer) keg fitting to Cornelius (Pepsi/ Coke, depends on what you bought).
I don't know of any place that will fill a five gallon keg with beeer for you, but you can buy almost anything in San Francisco. Maybe one of the microbreweries will think of your keg as a big growler, fill it from a tap, put the lid on and then you (having thoughtfully brought your CO2 tank with you) can pop 20-30psi into the keg right away so the beer doesn't go flat. I would be looking for a joint with 700 gallon fermenters walking distance to a college, and hoping they make a beer you like.
M2c
