Sending a Resume without having a target Job Description

BLUF: What do you send in when a company asks for your resume but does not have a specific job posting or description?

Often, and especially at startup companies, there will be many job postings for technical people and very few or none for other skills (e.g., marketing, sales, operations).  Many of these companies post something along the lines of "Don’t see what you’re looking for? Send us your resume and we’ll see if there’s a fit!"  These companies may be a great fit for someone who has relevant experience in these other skill areas.

What is the best way to go about crafting a cover letter and resume that will break through if you have no network-based connection into the company?  

Thanks!