Yes, according to DM's Guild
Since Mearls always disavows speaking with authority, here's someone who is speaking with authority, writing today (7 June 2016):
Thank you for contacting us regarding the DMs Guild.
You can write for any timeframe of Forgotten Realms. All material produced must be 5th edition, but the history of the Faerun at large can be used.
If you have further questions, let me know!
All the best,
Community Content Manager
If it ever comes up, the question I posted could be referred to as "Incident #55636"
For context, here's the text of the request I sent yesterday, 6 June 2016:
I understand from the FAQs and CCL that I should be writing material for 5e and no older editions. Does that mean that material must be written and set in the Realms in the current timeline? That is, ca. 1480-90? Or may I set my adventures in a past time in the Realms, as long as the mechanics I"m using are 5e?