I'm not sure if this can be done, or is even plausible, or worth the effort.
I have a custom match-making mod.
When searching for matches, it uses membergroup selection as mandatory match, and custom profile fields as optional matchs(most matches rank higher on search list).
Membergroup selection is a dropdown. Custom profile fields are checkboxes. This is the way I wanted it for simplicity.
Everything works perfectly. It would be best for you to see it in action before going any further at my non-active site. www.inspireromance.com/SMF click on "Personals".
I'm wanting to extend it by including perhaps adding area code to the search. I'm been thinking about ways to do that in a reasonable manner.
The simplest way I can think of, is a custom profile field that is a text area for entering in zip code. Then in the search list, make it an exception and it not appear as a checkbox selection(that's impossible), but instead as a text area so the member can type in a zip code that looks for matches. This text area would be at the top with the dropdowns.
The way I'd really like, is some freaking way to add zip code and member search "so many miles from" the zip code entered. I reckon that would require some heavy coding, or perhaps a zip code plugin or something.
Just let me know if I confused you. Is this something you think you can do? If so, I'll package the mod up and let you dig into it.