All my papers have alphabetical author lists, but an award application asks for author lists to judge contribution

If you are concerned, you may want to include more information about the culture of coauthorship in mathematics. For instance, you could enclose the following updated AMS culture statement.

As a mathematical "lifer," I want to be honest: the way that we handle coauthorship in mathematics does sometimes obscure the individual contributions of the authors. On the job market, this is often circumvented by getting letters from one's senior coauthors describing the contributions you made. You might consider doing that here, or that may be too heavy-handed for this case.

You could also talk to a mentor in your department and see what they think. They may have more experience with what happens at the university level.

Unless the CV has a fixed, required format, such as in an NSF proposal biosketch, you are free to include any additional information you judge to be pertinent.

Since they require the author order to match what's in the paper, then you should preface your list of publications for this CV with a quick note stating: "Mathematics publications use alphabetical order when listing authors because [reasons]." That will allow you to explain the issue concisely.

(Note that a number of other fields have similar issues—such as economics.)

I worked for more than a decade in a field where authors are always listed alphabetically, and the typical author list includes 500+ names. Individuals do get awarded in this field.

As it has been suggested, the contribution of a single author in such a paper is not officially recognized on a paper in this case, but it is unofficially recognized by the community. Most people in your field will know the work of somebody worth an award, either through letters of recommendations and word of mouth, or presentations, posters or seminars on the same topic of papers you published.

As you can afford only one letter for this award, all you can do is to highlight the public talks you have given related to the papers you list on your CV, and count on your reputation in the field. Good luck.