You could try going all the way to Dropbox and using the shared link to your file. Substitute "dl' for 'www' and get https://dl.dropbox.com/s/(YourEncodedDropboxAccountInfo)/myfont.css?dl=0. This can be inserted into your <link href> statement. The Workflow app can help you get the 'www' version of the link for you if you're strictly iOS based or a right-click on the Dropbox file on a Mac can also get you the shared link (I'm assuming a right-click also works on Windows).
There may be an Editorial workflow that can get this for you. There is one called 'Insert Dropbox Image' that uses the Workflow app to choose an image, obtain the Dropbox link, substitute 'dl' for 'www' and inserts the modified link into your document. You might be able to modify this workflow to suit your needs.