Last night I went to the library and into the journal stacks (which may not be available much longer) to re-read the article (Miron and Wilchek 1982 Analytical Biochemistry 126(2):433-435) on using spectrophotometry to follow coupling reactions involving NHS esters. When I consulted this article before, I was running a reaction in an organic solvent and attempting to extract N-hydroxysuccinimide into the aqueous layer of a micro-extraction. One thing that limited the usefulness of this technique in my hands was that there was a large value of the absorbance at time zero. This might have been due to the reagent itself or to an impurity in the starting materials.
I am not trying to beat a dead horse, but if there were some way to perform UV spectrophotometry in your application (I know almost nothing about SPR chips), the spectrophotometric method might have more than one use. One of their applications was to measure how much active ester was found on commercial gel designed for coupling to macromolecules, and I can imagine that this might be useful in your work as well.