Can a cost CLIN be utilized on a commercial contract to support unplanned yet necessary ODC and Travel cost?
You are not allowed to utilize a cost CLIN when using the commercial contract format FAR 12.303 and the SF1449).
Technically, FAR 12.207 is not silent on the use of cost CLINs. FAR 12.207(a) states "Except as provided in paragraph (b) of this section, agencies shall use firm-fixed-price contracts or fixed-price contracts with economic price adjustment for the acquisition of commercial items." [Bold and italics added for emphasis] In all instances in the FAR the use of "shall" identifies a mandatory action, it is not optional.
Note: FAR 12.207(b) discusses the use of a time-and-materials CLIN when all of those conditions exits. See the definition of "materials" at FAR 16.601(a). Perhaps if your acquisition meets all of the requirements at FAR 12.207(b) and your organization gets the approval, you could use a T&M CLIN instead. But don't forget to follow the guidance at DFARS 212.207(b) and DFARS 216.601(d) as well.