Hi - unfortunately I don't think there is any way to access these attributes of a post from MarsEdit. I don't believe that Squarespace has exposed access to these via their API (the interface MarsEdit uses to talk to it). If you discover otherwise I will consider adding support for editing them in a future update to MarsEdit.
These feel like the kinds of things that would be well suited to editing with MarsEdit's "Custom Fields" support, but as far as I know Squarespace doesn't support any custom fields via their current implementation of the API.