Photons have no mass but they convert to mass when they are absorbed by the elections on the surface of your body. You would have to weigh yourself before you begin to cast a shadow then again when your shadow fades out to calculate how much mass is displaced by your body casting a shadow.
Interesting. Using comparison measurement and appropriate calculation you could probably come up with something meaningful. But down the ages shadows have caused fear in those who believe they detect some as 'free-standing', otherwise unaccountable, shadows. Who knows, there may be invisible entities that absorb light, similar to the way invisible entities are reputed to absorb heat, causing cold patches in an environment. Yes, I'm talking evil entities.
Kittigate 