Why is a hard fork required to implement that? Client would only need to accept and relay partly signed transactions in the memory pool. If I receive a block that only requires my signature, then I sign it and can compare it against the target and broadcast the finalized block to the network if the target was met. Otherwise I sign the candidate with my key and rebroadcast the partially signed block.
Could be done in a soft fork, or am I missing something? I thought multisig staking is already possible in the peercoin codebase, because I know at least two PoS dev trees which have it, and peercoin was always ahead of them.
And its not about the block reward, its about casting votes, and with the recent increase in the price per 10k shares services like mentioned in the OP will soon be very desirable.