Hi there - yes, scheduling posts for Squarespace is possible using the method of setting the date explicitly from MarsEdit with Post -> Edit Date just before sending it to the blog. You can read more about this technique here:
There is a caveat though which is I think Squarespace still ignores the time zone information on the date that MarsEdit supplies, so after you send it, examine the scheduled time carefully to be sure it matches your expectation. You may need to adjust the date in MarsEdit to correlate to the GMT time you are targeting, to get Squarespace to record the date correctly.
Thank you so much for your answer and may I say I really love your software. I'm not using the SqSp online editor anymore. Thank you for the great work you're doing.