I'm pretty sure this is not possible, unfortunately.
I think it would be a pretty great feature myself with many different applications, but...yeah. AFAIK visibility is calculated from the camouflage (or somesuch) rating of the soldier in the config, which can be increased or decreased (ghillie-suited snipers have a higher camo rating, civilians lower. I believe); but again, not dynamically. If they had retained that one config-changing command they used in one of the betas, we might have had something, but since they didn't...
You could try messing about with setcaptive, see how it affects knowsabou? In a campaign environment, you could probably completely replace the spotted soldier with a new, identical soldier that you loadState and switchPlayer to - presumably the knowsAbout would disappear with the old soldier then too. But that's not a solution, really.
Wolfrug out.