In Mystic Homes, we have a rewards system for daily visits to the dream. We'd like this to be per-account, but this cannot currently be done through DragonSpeak, and this doesn't seem very likely due to privacy concerns.

Another method I have considered is using group membership to limit reward handouts, but there's nothing stopping players from joining the group on multiple alts.

It would be nice to have the option to turn off alt joining. This might be easy to do, as the button already changes when you're requesting an alt join. Going further, it would be nice to set a custom limit, potentially by group level. This wouldn't lead to any privacy concerns, as the only person being notified would be the person joining the group.

Use a bot and store the visit data to a database. That will work fine, and it is not unwieldy. Seeing how Phoenix Speak has imposed limitations, I doubt that there will be databases for each user available without limitations at any time in the future.

PhoenixSpeak already does a fine job determining visit data, that isn't the issue. The issue is determining who is an alt of who, which is technically a privacy issue but a valid concern for dreams that offer incentives to players. The best we can do right now is offer incentives to group members only, because then at least they have to go through an additional step. The problem is that many alts from the same account can join the group, so it isn't really that discouraging. This is potentially problematic in another sense, in that I'm pretty sure groups have member limitations (correct me if I'm wrong)... which could be eaten up by alts.

Do you not run into PS issues when you run out of users? Or is that just not reasonable to worry about with how few people visit dreams these days? Alts are completely undeterminable.

I've never been alerted to running out of membership slots on the group itself, and we have never run out of slots or space in the PS character database. I occasionally clear out unused accounts, but we are maintaining 324 characters just fine right now with plenty of room to spare.

Aside from being able to know if an alt belongs to a particular account, yes, alts are indeterminable, which is an issue for our rewards system, thus the suggestion.