Experiments have a weight configured, but adding/removing experiments does not change the weight of the capsule.

I believe the IPartMassModifier interface is what you want.

THANKS! All done in mw_dev branch.

EDIT: including for OMS

For future reference: http://forum.kerbalspaceprogram.com/index.php?/topic/137059-changes-to-ipartmassmodifier-ipartcostmodifier-and-ipartsizemodifier-in-build-1209/