My first thought is the same as Tracy's - this is more-or-less what "person" was designed to be by aD back in the original 4.0 design days (I'm basing this on my reading of the datamodel over far too long a time, not any special personal information).
Also be aware that Dave and Jun are both working on general-comments (the partial work is in contrib) ... is there some possibility of enabling "person" comments and improving the UI or possibly providing an optional "comments lite" UI or the like? General-comments is perceived as needing improvement, which is why Dave started on an improvement project (and why Jun recently joined him), so if at all possible the very best solution would be to fix this general facility.
And comments by unregistered persons such as you describe is an option that other packages might benefit from. If it's good for blogs (are we sure it is? that's another question, I guess) it should be good for photo-albums and similar packages.
