Yes, this is possible, the support team can provide you with a copy of your Mercurial repositories.
I created a support ticket for you using the email of your community account. You should have received an email with a link to the support ticket. Just in case you haven't received the email, please feel free to let me know and I can post the ticket URL here. The support ticket will be visible only to you and Atlassian staff, no one else can view its contents even if they the URL.
One of the Bitbucket Cloud support engineers will respond to the ticket and provide you with a copy of your Mercurial repositories.
If you have any questions, please feel free to let me know.