But how do you deal with staggered arrivals ? (like lets say you have 20 contributors to a node. They won't all turn up at the same time - you need some kind of opening and closing gate that ensures that the collateral is fully populated during a continuous period).
I wouldn't. As I said shared MNs service on the protocol level is stupid, let crowdnode manage that. The time_locking I was referring to was the individually run masternodes, at the time we are filling out the protx ... RPC call, there should be another parameter for the duration of the stake, 0,1,2,3 in years default 0 where if you commit to a length of time the network pays you a small bonus over and above your usual reward and thus improving DASH as a SOV. Don't worry, this idea is hated by all including the devs and will never come to DASH. Good news is other tokens have built in and we can diversify into them for SOV and keep DASH for the payments.