What I think we're mostly doing in the regression tests is shutting off every relevant type of plan except one. I theorize that what we actually want to do is tell the planner what we do want to happen, rather than what we don't want to happen, but we've got this weird set of GUCs that do the opposite of that and we're super-attached to them because they've existed forever.
So rather than listing all the things we don't want to happen, we need a way to force (nay, highly encourage) a particular solution. As our costing is a based on positive numbers, what if we did something like this in costsize.c?
Cost disable_cost = 1.0e10;
Cost promotion_cost = 1.0e10; // or higher or lower, depending on how strongly we want to "beat" disable_costs effects.
...
if (!enable_seqscan) startup_cost += disable_cost;
else if (promote_seqscan)
startup_cost -= promotion_cost; // or replace "promote" with "encourage"?
