As for slinging them - Pretty sure the DG regs apply to slung loads as well (albeit perhaps with some easements)
Taken from
CAP426 :
"The rules applying to internal DAC loads do not necessarily apply to those carried externally. USLs pose less of a hazard to the helicopter and crew because the loads are further removed from them. Furthermore, USLs can be jettisoned immediately if required. In any event, the helicopter captain must be advised of any DAC aspects relating to the load that he is tasked to undersling."
Whether this applies outside CAAland, and quite what "....do not necessarily apply ..." means, I don't know.