I'd recommend dropping "copywriter" from the description, as there is (in my experience) no interest in paying copywriters in startups (perhaps some minor interest if you can couch it in being killer at SEO, but even then, not usually).
Startup marketing is typically an "army of one" job, particularly early stage.... so, yes, graphic design chops and a working knowledge of HTML will allow you to create marketing materials (email newsletters, landing pages, Facebook tabs, blogs, etc.) without requiring you to take design resources from the product side. This will speed things up and increase your value immensely.
Overall, I'd work on branding myself as a marketing hacker - someone who can handle marketing top to bottom, grow the business, and become the leader of a larger team should things take off.
Hope that's helpful.