Is a Theravada Buddhist permitted/able to take a Bodhisattva vow? This is a sister question to this one.

Suppose a Theravada practitioner decided they wanted to take a Bodhisattva vow, could they (and remain Theravadin)? I guess there are a few of components to this question:


Does Theravada have scope for individuals to even make the Bodhisattva choice at all when the time comes? 
Even if it does, does it then allow for individuals to make a promise, in advance -- i.e. to vow -- that they will make that choice at that time? 
Finally, even if Theravada sees both choice and vow as being possible, does it regard people who make that vow as remaining as Theravada practitioners, or would they be seen has having moved into Mahayana?
The term theravāda means "doctrine" (vāda) of the elders (thera). It generally refers to the teachings found in the Pali Canon, commentaries, and sub-commentaries. Anything laid down as doctrine in this large body of literature should be considered as theravada.

Does Theravada have scope for individuals to even make the Bodhisattva choice at all when the time comes?

The Buddha did not, as far as the theravada goes (and as far as I'm aware), teach his followers to become Buddhas, since his focus was on leading beings to freedom from suffering as soon as possible. It is acknowledged as a valid path, however, since that is obviously the only way a Buddha can arise. In many places the Buddha talked about how great was the arising of a Buddha in the world, for the benefit of many.

Even if it does, does it then allow for individuals to make a promise, in advance -- i.e. to vow -- that they will make that choice at that time?

Again, there is no doctrinal teaching on the matter; the closest you'll get is in the Jataka commentary, translated in Buddhism in Translations, which talks about the perfections and characteristics of a Bodhisatta.

Finally, even if Theravada sees both choice and vow as being possible, does it regard people who make that vow as remaining as Theravada practitioners, or would they be seen has having moved into Mahayana?

The theravada doesn't recognize the term "Mahayana"; it recognizes three paths:


sammā-sambuddha - one who becomes enlightened by themselves and is able to teach others to become enlightened due to their limitless knowledge.
pacceka-buddha - one who becomes enlightened by themselves but is unable to teach others to become enlightened due to their limited knowledge.
sāvaka-buddha one who becomes enlightened through the teaching of a sammā-sambuddha.



  While thus I lay upon the ground, Arose within me many thoughts:
  "To-day, if such were my desire, I my corruptions might
  consume.
  
  "But why thus in an unknown guise Should I the Doctrine's fruit
  secure? Omniscience first will I achieve, And be a Buddha in
  the world.
  
  "Or why should I, a valorous man, The ocean seek to cross alone?
  Omniscience first will I achieve, And men and gods convey
  across.
  
  "Since now I make this earnest wish, In presence of this Best of
  Men, Omniscience sometime I'll achieve, And multitudes
  convey across.
  
  "I'll rebirth's circling stream arrest, Destroy existence's three
  modes; I'll climb the sides of Doctrine's ship, And men and
  gods convey across.
  
  -- The Story of Sumedha