I have been used a commercial product of anti-thrombin antibody, and used a fluorescence-labelled secondary antibody for the detection. It worked very well.
you are right, ie. activity will be another thing - but hard to analyze, if a (suspected) elevation is locally restricted. I really like the studies with ACPP by the Tsien group.
Using hirudin as a negative control seems applicable and not too expensive to try out. Maybe if it is working one could even use an anti-hirudin antibody as positive control.