I noticed that pt.stackexchange.com leads to a 404 page; it becomes annoying to have to spell out the longer URL. Even though now, in private beta it won't be very useful, as you need a special link with a key to be able to enter, I feel like it would be better once we go public. Could it redirect to portuguese.stackexchange.com?
My opinion stands on the borderline, but the SE community in general would more likely go against it. Indeed, having an abbreviated version could save a bit of typing (only a small amount of us have taken a course on proper, professional typing, eh?).
On the other hand, no other language site on SE has been gifted with an abbreviated form of a subdomain, as @Earthliŋ mentioned. Furthermore, attempts to standardize site names were made, suggesting the use of abbreviated language subdomains for sites under the same scope of questions as the originals, but in that language (Portuguese Stack Overflow is currently under pt.stackoverflow.com). I don't think
pt.stackexchange.com will ever happen as a translation of Stack Exchange, but we never know what future decisions will take place. It could probably hold a list of questions from all Portuguese SE sites later on.
meta.pt.stackexchange.com would never make sense, though.
And most importantly, the current site federation model for SE already poses a generous amount of subdomains under
stackexchange.com. Only sites creating a very large amount of traffic (Stack Overflow, Super User, Server Fault, ...) or those in which a special partnership was established (Ask Ubuntu), have their own domain. The "United States of Stack Exchange" approach is simple, but does have its drawbacks.
I could not find anything specifically about subdomain synonyms on SE Meta, but since we already have such an aggregation of sites under the same domain, my hunch is that the idea will be frowned upon.