ambient light rejection in absorbance sensor
I think I would forget about analog filtering altogether.  

Your signal will be sampled digitally, and digitally filtering is a lot easier: sample at a rate high enough not to miss the 100 &micro;s pulse, like at 20 ksps. A full cycle will then have 1 to 3 high level readings from the pulse, and the rest the ambient offset. Average the high level pulses and subtract the average of the low level ones and you're done.  

Analog filtering may deteriorate this, since you won't get the nice rectangles with relatively flat level high, and ditto low.  

The ideal would be that you can synchronize sampling with the pulses, so that you can do a quick series of samples in the middle of the pulse, which you can then average.