I get that they don't currently attach completion rewards to UGC missions, because they don't want people making Hello World missions and grinding them for free SP. However, Foundry has some fairly tight boundaries for how far afield you can go in setting mission objectives, and it seems like these limits could be used as a "feature" for letting authors make missions that would qualify for a completion reward.
For instance, you can't place kill encounters that are anything other than the usual Gang of Five mobs we find in all the standard mission chains. It seems like Foundry could reasonably allow a mission to qualify for a completion reward, as long as it included a minimum number of Kill Enemies encounters. This would suck for author's making non-killing missions, because theirs might be more fun and require more time or effort to complete, but would not qualify for a reward. But at least it would be something.
You could still put 25 mobs in one big room and have a mission where you just go in and mow them all down. But that's not so different from what the standard issue explore missions involve today, so there's no principled reason not to allow some reward for it. And requiring a minimum number of kill encounters seems like the simplest bar for the tool to enforce.
The reward should be a standard, set amount of SP, BOFF SP, and merits. It could likely never include emblems, because even if there were a way to flag a UGC mission as "daily," there's no limit of the number of unique daily missions that could exist. And since emblems = purple gear, any UGC that produced emblems (or any other merit currency) would be relentlessly exploited by gold farmers.
I would love to think there is some way to qualify a mission for a standard 10 DXP completion reward as well, since the diplomatic missions are far the shallowest and most tedious puddle of the content well. But I'm not sure what sort of rule could rationally allow this. The nonkilling objectives are just too easy to minimize by putting five "interacts" or "contacts" side by side in a room, requiring no action at all, that I'm not sure how you could guarantee even a minimal amount of effort or time to complete them.