When the ringct branch is finished, will it make m-of-n multisig easier or harder to implement?
RingCT will make multisig easier to implement, as the planned multisig system relies on RingtCT building blocks.
Contrary to the Bytecoin multisig, Monero's will not be limited to mixin 0 transactions (in fact, Monero banned mixin 0 transactions in the general case since it is harmful to privacy, so cannot even use that system as it is).
In theory multisig was "simple" to implement in Monero, at least the way the Cryptonote whitepaper describes. It had never been implemented though.
There is ongoing research to implement multisig on top of RingCT and you can find a draft MRL here.