@Ram

“Download Board” →  What would you like them to download? 

Just the text from the items in the board? A backup to the board - so that they could load it in later after creating an account? 

You could pretty easily offer your users a downloadable CSV of all the data available in the board, translating it to a meaningful set of rows and columns (much like what Miro offers in the ‘download csv’ feature.    Here’s what I can imagine being possible. You can’t make a ‘plugin, because users in ‘guest editor mode’ can’t access plugins.  You have to make a separate website with a form field called “board URL” -- if you send those users there after they have made their Miro board content, they can give your web-app the board URL, I’m pretty sure you should be able to do the rest with API (assuming that your webapp uses your team-specific (given at install) Oauth Token to retrieve data from the boards that are made under that team). You may need to do this through a little proxy server / cloud function to hide that Oauth from your web-app’s front end -- easy enough. 

I haven’t really looked into the format / file structure of a ‘board backup’  / ‘.RTB’ file.  But, hypothetically, if no funky encryption/obfuscation is taking place there, I presume you could reverse engineer how to write to that file structure and create a workable board backup file. 
 

Cheers,

Max