Is the term "heteroatom" reserved for exclusive elements?
The IUPAC Goldbook provides no 'official' definition of the term heteroatom,and as such you're somewhat free to use (and abuse) the term as you see fit. 

Since no IUPAC definition exists, it may be helpful for you to define what you plan on calling a heteroatom at the beginning of whatever you're writing, to inform the reader (common practice in scientific literature for poorly defined terms). Clearly an organic chemist would have a very different definition to someone involved in purely metallic synthesis. 

I would however, point to several other IUPAC documents in which, whilst no 'definition' of a heteroatom is given, they do suggest lists of possible heteroatoms. For example, the following statement from the IUPAC nomenclature guidelines which includes many elements (including metallic ones, possibly going against your suggested definition).  This priority order of heteroatoms is repeated multiple times. 


  P-22.2.2.1.3 If two or more kinds of heteroatoms occur in the same name, their order of citation follows the sequence: F, Cl, Br, I, O, S, Se, Te, N, P, As, Sb, Bi, Si, Ge, Sn, Pb, B, Al, Ga, In, Tl, Hg.