missed the message of registration.
There are a few efforts related to the bootstrapping of the new chain that are intended to address ERC20 holders who have no registered address. For instance, https://github.com/EOSIO/eos/pull/2669 which intends to put all the tokens in a holding account so that they can be distributed later once an appropriate claim process is approved by the community. I must stress, the capability of this is in the community's hands so, it is up to the community to decide how this is handled.
registered and lost their EOS wallet private key
Again, facilities exist for the community's approved system contract to recover these funds. However, it is far trickier to prove destruction/loss of private key. In the end, the community will need to come to consensus on how this case should be handled. From a technical perspective it is handle-able. The privileged system contract can act as an "root" account and issue transactions.
Including issuing a transfer from one account (the lost key) to another. The expectation is that this contract severely limits the cases where such privilege is used.