You can now set the incrementProperty when using DFP IMA, as shown in the list here: http://support.brightcove.com/en/docs/integrating-google-dynamic-allocation-and-dart-publishers
This allows you to have some key/value for targeting multiple ads and has been tested/proven and is in use by several publishers.
Another way to go about this is to use a feature of the requestAd() API. Set only one ad to be requested for the ad policy but then listen for the adStart event. Check that it's a midroll, and then call requestAd with the second parameter set to true:http://docs.brightcove.com/en/player/com/brightcove/api/modules/AdvertisingModule.html#requestAd%28%29
This allows you to put the ad in a queue, effectively making it partof the same group of ads. And the first parameter of requestAd allows you to set any key/values you want for targeting.