Entity Reference to fields
Ok, I've thought of a solution that somewhat inelegant, but it works

On the Equipment content type, a hex/dec signifier field is added--some sort of a select box (maybe a taxonomy selection box) where the user will select if the serial number in hex or dec.

When the form is submitted, a custom rule will check for the presence of the select box, convert the value in the "title" to the other numbering format, and then append that value to the original value in the title field, so that the title field will contain both formats of the number. After everything is said and done, the Equipment titles will all look like "1234567, 12D687".

Now, when the user creates an order and searches for existing serial numbers using the Inline Entity Form, as long as you have it set to "autocomplete", it will find the correct piece of equipment and associate it with the order.

Still, it would be nice if Entity Reference auto-complete would work with fields from those entities and not just their titles.  And, I'm still in needs of some sort of module that will add a hex converter to Rules.

Let me know if anyone finds better solutions!