Although I guess that's not a very good solution for the average user.
Definitely not.
The existing notifications tab is kind of a poor man's version of this, for me -- I remember things that I've said or replied to, and they're in notifications, so that's how I find them again when I want to talk about them.
This is interesting! I don't really use my notifications like that. I do go to my profile to look for my recent stuff though. hmmmm

Please share whatever you come up with! I find you guys invent better solutions for yourselves via hacks than whatever we'd come up with.
A few stackers put their first item number in their bios like, "I've been on stacker news since #1120" so we added that to everyone's profile. Likewise, seeing everyone organize their favorite posts on their bios has us thinking of building a feature to better support that.