Right, it means it wouldn't be closing ports at all, they'd all be open, it just probably wouldn't be giving out work on the ports that would normally be closed (and also based on those weight factors). I assume sgminer sees those as "down" or not giving work fast enough, and ignores them (otherwise, what would it work on?). So that shouldn't be too tough actually.
Again, haven't started the implementation, but been thinking about it a good bit

Here is the NH implementation:
https://nicehash.com/multialgo/#finetuningSince it's based on the password, authorization will fail on all ports except the one that's most profitable ATM. This works not only with sgminer5, but with some additional scripting with other software (with some loss of performance due to slower switching of course), so even though it's quite hacky it's still a decent stopgap solution.
In addition to allowing to set custom multipliers, they also allow to set the power cost, and exclude certain algos - e.g. if I don't have my rig set up for X13 I can set f4=0 and the pool will authorize me on the next most profitable algo.