A couple of additional thoughts to throw-in, building off of Dan and Jim's comments (and thanks to Peter for continuing to stimulate this conversation and for integrating it with the thoughtful conversation you and Meira have been having)...
One issue is the centrality of civic identity. That is, where on the hierarchy of an individual's various identities does civic identity fall or should it fall? One might argue that having civic identity as the central identity is not necessary for a healthy individual or a healthy democracy, but having civic identity present somewhere on the hierarchy probably is important; with the qualifier that an individual recognizes the importance of being involved to some extent in the democratic process, broadly defined. Thus, to one of Peter's points, an individual could more strongly identify as an artist, a spiritual person, a parent, etc.., but that a civic identity could still exist within the person and not crowd out the other identities (indeed, I'd argue that the identities most likely complement each other and result in different expressions, for instance, of civic actions).
Another issue brought up is whether a scale of civic identity is appropriate considering the diversity of ways that a civic identity might be expressed. I think you've probably figured out that I'm biased toward developing such a scale, so take this with a grain of salt... Religious, gender, ethnic/racial, and vocational identities could also be considered to be constructs with a diversity of expressions. However, for better or for worse, these constructs have been operationalized and subsequently have had scales developed around them. Ethnic identity is a good example of this. There have been more general conceptualizations that are supposedly representative of all ethnicities (see Phinney and her disciples) as well as more ethnicity-specific theories and measures (see William Cross and Robert Sellars for examples of Africentric identity theories and measurements). On the one hand, these theories and measures complement each other, such as the identity search component of Phinney's MEIM and Sellars' Centrality component of his MIBI. On another hand, they are opposed to each other conceptually (e.g., Phinney's is not context-dependent whereas Sellars' is dependent on the context surrounding anyone who is African American). All of the measures have strong theoretical underpinnings and psychometric properties, including concurrent and predictive validity. At the very least, these theories and measures have pushed the ethnic identity field forward empirically as well as theoretically since they stimulated discussions about the differing theories.
The same could be said of the civic field. There are differing theoretical opinions about civic engagement, but that does not necessarily mean that putting a stake in the ground about the concept and measurement of civic identity (or civic identities, if that's a more appropriate conceptualization). I would agree wholeheartedly that developing the theory needs to come first, since researchers can use statistics to manipulate data and then fit a potentially erroneous theory to the data.
Regarding whether civic identity is context-dependent, I agree with Jim and Dan that civic identity is context-dependent, or, maybe more accurately, the interplay between the individual and the context (person-context interaction), which is the basis for developmental processes, in general.