Can concretions form in non-sedimentary rocks?
Not really. Concretions are features of sedimentary rocks almost by definition. Here's my go at a definition; it essentially presupposes a sedimentary rock:


  Concretions are spatially discrete zones of above-average (compared to the rest of the rock) cementation by authigenic minerals. Common cements include calcite, quartz, siderite, and pyrite.


Concretions form during the process of diagenesis. However, diagenesis and metamorphism are really on a continuum of 'burial effects' — increasing temperature and pressure, compaction, changing porewater chemistry, and so on. One could almost call diagenesis a metamorphic facies. Concretions can start to form immediately after burial, and grow during burial as long as geo- and electro-chemical conditions allow, or until the pores are completely occluded. According to some workers (e.g. Bjoerkum &amp; Walderhaug), total occlusion usually happens somewhere between about 150 °C and 200 °C. By this time, clay minerals have started altering and we are into low-grade metamorphism (zeolite facies).

Conversely, I think most so-called soft-rock geologists just lump all alteration into 'diagenesis' (I know I did until I started working on geothermal reservoirs). So, if you're prepared to explain yourself because it sounds weird, you could claim that concretions form in metamorphic rocks. 

Footnotes


I've never seen one, but I don't see why you couldn't have concretions in volcaniclastic rocks (which I count as sufficiently like sedimentary rocks to warrant no more than a footnote).
If you were desperate to call something else a concretion — a mass of phenocrysts in a granite, say — you could always broaden your definition of 'concretion'. We geologists love arguing about the definitions of words!