I think I ended up firing out why it doesn't work. The actual challenge should read "Get Spot Assists on objectives with Recon gadgets as Recon". It's not that it doesn't register, it's that it only registers spot assists. It's still a pain in the **bleep** to get 300 assists though. Even if I spot the entire round next to a medical crate and keep the entire enemy team spotted constantly, I may only get around 60 spot assists, but only about 20 of them are actually on an objective. Having to sit an entire round spotting for less than 10% progress seems insane to me.
