My question is whether or not this is an allowable contracting practice.
Federal Acquisition Regulation (FAR) 16.504 describes the circumstances in which an indefinite quantity contract could be used and expresses a preference for multiple awards. FAR 19.803 describes the process to be used by the SBA and the agency for selecting acquisitions for the SBA 8(a) program. It further states in FAR 19.803 that the SBA can identify specific requirements for a particular 8(a) firm or firms and asks the agency contracting activity to offer the acquisition to the 8(a) Program for the firm(s).
You should work with your SBA office to identify 8(a) firms for this requirement and establish a basis for this type of contract. It appears to be allowable but it is always advisable to review your requirements with your legal office.