Out of curiosity I just spoke to someone who is well versed and involved in these matters.
In general when a HMR is granted to the husband, he (the husband) deposits a get in escrow to beis din. When the woman is able (for example, she wakes up from a coma etc..) the beis din will give her the get.
As with every other complex issue a competent Rov should be consulted.
I believe R’ Moshe ZTL wrote several Teshuvos regarding giving women the ability to remarry after World War 2 when it could not be verified if the husbands survived the camps or not. I think there was a reference to a mechanism allowing remarriage when the husband could not give a GET.