This might be of help (the switch-on guide):
http://pc.mtld.mobi/documents/dotmobi-Switch-On!-Web-Browsing-Guide.html
It doesn't say that you can't use third-level domains (i.e., "subdomains"). In fact, it states that you should "Identify national variations of dotmobi sites by using the corresponding country code top level domain identifier (ccTLD) as the third level domain identifier." (Note: using the ccTLD like this is not mandatory but is in the list of "highly recommended best practices.")
The only restriction is that, when a website is accessed through a third-level subdomain or through just the second-level domain name, "the response must be encoded in XHTML-MP unless the device accessing it is known to support an alternative choice of markup." This is a mandatory registrant rule.