Someone who does a lot of experimental hybridizing pointed out a method to greatly reduce insect cross-pollination from nearby plants and neighbors, without spending lots of time hand-pollinating.
First, you have to cover the heirloom or valuable plants that you want to conserve with tulle or fine mesh. Cover them really tightly, like by laying PVC pipe or soil all along every edge. Remember that insects can burrow if they really wnat to reach a bloom.
Let them grow and bloom under the mesh, without any pollination at all, until there are lots of blooms ready for insects.
Wait until nightfall, when all or most pollinators stay in their nests.
Between sunset and sunrise, set yourself up to have all your pollinating done early the next morning, on the bees' first few flights, when they have a minimum of other pollen on them.
At night, remove the mesh from your prized heirlooms. If you have anything in your own garden that might cross-pollinate it, cover that tightly to keep insects away from it tomorrow. Or dead-head every bloom and bud that's about to open.
If it's an option, time this for a time when the nearest neighbors' pollen sources are not in bloom, or have recently been dead-headed or covered with mesh.
I assume that pollinators rise with the sun, but in any event, they will find your recently-uncovered blooms and go into a pollinating frenzy very early in the morning.
Depending on whether you want them to be as fully-pollinated as possible, or as free from cross-pollination as possible, cover them back up early in the morning, or after an hour or two.
Then re-cover the heirloom with the tulle that was originally on it, and seal the edges tight, after letting the bees out. Now you can uncover any other plants that you isolated temporarily.
BTW: as soon as you drape tulle or mesh over some blooms, it will pick up some pollen. If you re-use it on another plant right away, it will transfer pollen to the next plant it touches. Unless insects really have to work hard to reach its stigma, that mesh could be a cross-pollinator. So wash the tulle or mesh between re-uses, or at least let them be rained on and give the pollen time to go stale.
