OK, you talked about a DAC taking care of the measured signal, so I guess there is a Microcontroller around, too. Use a PWM output and a low pass filter to make a little DAC to control the light intensity. In the µc, you can play with the time constant of the LED intensity regulator however you like it.. even measuring it once, keep it on a fixed value until the fixed value doesnt work anymore because the ADC is over/underfed.
Fixed intensity with a resting test person can tell you a bit about the tendency of the blood pressure rising or falling.
Yeah, kids are great - mine always wants too see what I am doing, but at nine months still quite easy to keep off of the devices i am playing with