Broadly speaking, sure, but even considering only northbound trains there are 3-4 arrivals per hour at Millbrae during commute hours. Reading across the Millbrae row at http://www.caltrain.com/schedules/weekdaytimetable.html :

am:
5:35 6:10 6:26 7:01 6:47 7:19 7:26 8:01 7:47 8:19 8:26 9:01 8:50 9:19 9:47 10:20 10:53 11:20

pm:
12:15 1:15 2:15 3:20 3:43 4:12 4:45 4:59 5:07 5:45 5:27 6:01 6:09 6:45 6:28 6:59 7:07 7:43 7:55 8:35

also 9:35 10:35 11:35 but those are after the 9pm transition.

Of course I am viewing this selfishly because I live on the peninsula but it'd be nice if the connection were less awkward.

Anyway given all the parameters it seems like a dedicated shuttle or spur line would make more sense.