Is there anything I can use in pumpkin custard to replace salt?
You can certainly omit salt from a pumpkin custard recipe. It's there as a flavor enhancer, to provide contrast to the sweetness. The texture of the custard will be just fine without it. Desserts without salt can sometimes seem insipid, but the mixture of spices in pumpkin custard should prevent that issue. I wouldn't add any additional spices.
Sometimes I forget to add the salt to a pumpkin pie, and no one notices the omission. Pumpkin pie filling is basically the same as a pumpkin custard.
Some pumpkin custard recipes have a pecan topping, which would add some additional texture and flavor. The pecan topping is a mixture of pecans, brown sugar and melted butter, sprinkled on top of each custard before baking.
Or you could make candied pecans as an optional topping. As with the custard it will be fine to omit the salt. If you're serving guests and not all of them are on a low-salt diet, you could make two batches of candied pecans, one salted and one unsalted.

edit: as chasly mentioned, if you are on a low-sodium diet there are various salt substitutes that don't have sodium, such as potassium chloride. I can't specifically recommend them because I haven't tried them. Potassium chloride is often described as having a metallic taste. The benefit of having some saltiness might be outweighed by the metallic taste, but it might be worth experimenting. You could split a batch of unsalted custard and add salt substitute to one half.