Re: filter at the telco level?
In the UK, telco's filter over-the-air content all the time (and are in fact required to do so). You can request that they turn the filter off, but you have no legal comeback if they filter something you want.
It's just false to assert that an ISP would somehow become liable for anything if they blocked malicious MMS traffic. Particularly since this is exactly analogous to efforts to block what used to be called "phone phreaking": techniques to misuse telco systems to achieve nefarious ends.