Nothing else will pollinate a pecan. Some are partially self fertile but most need pollination for much of a crop.

In pecans there are two pollination groups. One sheds early and flowers late. The other is opposite. There is some overlap but usually you want an early and a late pollen type. Since yours is a seedling, you don't know what type it is and therefore can't chose a reliable pollinator. If you buy a type 1 and type 2, you should be covered. If you get more seedlings, 3-4 should have all bases covered.

slinger(and others - I'm not just singling you out),
If you'll indicate where you're located, folks might have a better shot at giving a reasonable answer to your questions. For instance, if you're located in just about any of the Gulf Coast states, there are probably enough pecans - native/volunteer and grafted varieties - in the local landscape to accomplish pollenation of your seedling pecan - if there are other mature pecans within 1/4 to 1 mile, they'll get the job done.

That said, another pecan(or 2 or 3) will be your best bet, if there aren't others in the area - if you don't have room, see if you could 'give' your neighbors some to plant, and you'll all benefit.
I'm not above doing some 'guerrilla' planting(nod to Joe Real)- surreptitiously planting seedlings/seeds/nuts of desirable fruiting plants in out-of-the way spots in the neighborhood - fencerows, vacant lots, etc.
Pecans and other hickories will cross-pollenize - there are a number of 'hican' varieties that have been identified and selected - mostly hybrids with shellbark, shagbark, and bitternut hickory - but I don't know how effective the hickories would be with regard to ensuring full crops of nuts on your pecan.

I'm considering a pecan in our yard, but we'd be the only one in a 15-mile radius.

Fruit trees are multi-grafted all the time, and those can be self-fertile by cross-pollinating among their own branches. Has anyone considered doing this with nut trees? Are they not sold because nobody's doing it commercially or is there a biological reason that it won't work?

There is no reason that a multiple graft on pecans won't work. If you have limited space, it would be a good idea. However, if you have limited space, what are you doing with a pecan tree? They are huge.

Commercially available trees with several varieties grafted are mostly for gardeners with very small gardens and no room for multiple trees. No one with any sense tries to grow a pecan tree in a 10 square foot back yard, so there is probably very small demand for multiple variety pecan trees.

But, you bet, find a compatible pecan and graft on a couple of branches. You can buy grafted pecans (for mucho money) so the trees graft just fine.

Yes, a Hickory tree will cross pollinate with a Pecan tree.