Hi Paul, do you mean as one off custom development or available for re-sale? I assume you mean the latter. That I have not seen. I know there are quite a number of extensions for Milestone and Genetec but I do not know how often or readily available they are for re-sale from third parties.
Converting software from internal use to resale should raise a number of significant questions which may not make it past marketing, legal, or management because of risk and costs incurred:
Short and long term installation and support requirements
VMS configuration differences break it
VMS upgrade breaks it, and your organization doesn't implement every VMS upgrade
Multiple potential liability issues: merchantability, serviceability, failure to function as expected leads to customer losses somehow, ...
With widespread reviews to internet audiences, unanticipated adverse experiences with your software can potentially affect your corporate identity adversely
If software is your business area, you have widespread experience across multiple configurations of the particular VMS, you understand other hardware and software configuration issues, you have legal and technical support staffs, etc., such resale might already be well aligned with your business' selective advantages.