Marv - Good captions are eternal, so long as they're written according to the standard. And Photo Mechanic does indeed write IPTC metadata properly.
So, "a flag waves in the breeze, wherever, whenever, and however, blah, blah." will keep you in good stead in any future DAM, as long as it is standards compliant.
Keywords are another matter. Despite their popularity (and rampant abuse) they should be treated as an enhancement to good captions, not a replacement. Keywords might act very differently in any given DAM or even in different collections. That said, a properly applied, standards-compliant keyword or two might help here, too. Something along the lines of "followup", or "notfinal".
And for flagging (forgive the pun) for future follow-up, there are several purpose-built options. Among them, ratings and color labels stand out as options that will travel with the file to other systems.
The "[" and "]" characters are forbidden in filenames in some current filesystems and who knows about the future. Having them in filenames of precious assets would make me very nervous.
If you want to, in Photo Mechanic, you can copy your filenames into the caption (or any other IPTC field) with a template and a variable. Then Photo Mechanic's Find and Replace command will search for and replace square brackets in filenames. Be sure to back up files before doing this, of course.
Find will also work to find files in any single contact sheet, which could include whole directory trees up to some sort of size limit. So, up to that size limit, it would find square brackets in filenames or in IPTC fields.
As to indexing square brackets - I have no opinion on whether it's doable or desirable.
HTH
-Carl
