You'll need to do this in the code-behind.   From memory:

var metaTag = new HtmlGenericControl();
metaTag.TagName = "meta";
metaTag.Attributes.Add("name", "keywords");
metaTag.Attributes.Add("content", CurrentPage.Property["Keywords"].Value.ToString());
Header.Controls.Add(metaTag);

Thanks Deane, I have used the above as a basis to embed the meta data into a user control that sits within a master page...works a treat.