We got an email from the US Consulate in Chiang Mai today telling us that immigration officials have been given a directive that gives them the authority to approve "extensions of stay for certain foreign nationals, including US citizens." I suppose that's what they do anyway, but it sounds like this means they have more latitude now. We're hoping this means you don't necessarily need a letter from your embassy, especially since A) you have to make an appointment to get one here in Chiang Mai and B) appointments are booked through March 31.
TLDR : Answer Summary
The US Consulate in Chiang Mai has informed that local immigration officers now have greater authority to approve extensions of stay for certain foreign nationals, including US citizens. Some users shared their experiences and expressed concerns about the requirement of a letter from the US Embassy due to travel restrictions and limited flight availability. Currently, it's unclear if the embassy will issue such letters, which are necessary for applying for extensions.