Comrade Admiral Yeah, it's pretty much impossible to swallow your own tongue, what with the tongue root and epiglottis being positioned as they are.
Blocking off the pharyngeal cavity is definitely possible though, as we see here. This guy can probably make some fucked up consonant sounds that are generally considered to be impossible by phoneticians.
fluffy He should submit that for Microsoft's "I'm a PC" campaign.
"I'm a PC, and I can swallow my own tongue. MMMMMMMNNNNNNNNNN"