"We don't have to follow no steenking Best SEO Practices."
... and come to think of it, subdomains go from right to left don't they? So it wouldn't be
user dot function dot domain dot tld
function dot user dot domain dot tld
Honestly I think you're better off with
In fact to me this looks more like a "real" domain name, which is surely what they want. Or, heck, let the users get their own domain. If they have a reasonably uncommon full name they can probably still get it for $9.95. Or even unreasonably common: I detoured and found johnsmith dot com just sitting there.