That is, create a .txt file with the same name as the file you want tagged (but perhaps with "_tag" at the end) and put the tags and keywords in that .txt. I think any indexing search tool will find those tags and due to the same filename you then find the file you're searching for. If you have a consistent and unique format for your current tags in filenames that a script can isolate (like using [tag1 tag2 tag3] only for tags) then you could probably make a batch script that migrates to the ".txt system".
It definitively is an interesting strategy.
I use a similar one for all the software I download & for some archives – and to help me be consistent, I now use a simple shell extension that was recommended by at least 2 different people here : it allows me to create *.TXT notes on the fly when right clicking on a file ( http://www.moonsoftware.com/freeware.asp
: look for FileNote).
I’ve tried to do that for files also in the past and I disliked the fact that every single file was doubled : always had to make sure that I moved both files when I had to move them around, had to check both files when I renamed them, etc. I guess it’s not as bad as it sounds, but these little irritants stopped me from using this technique. Maybe there could be a way of doing it in a way that’s much more simple and automated. But… I haven’t thought about it.
Maybe you (or others) would have suggestions?
[edit : forgot to add an answer to Darwin's suggestion!]
@ Darwin :
I use Endnote too, but I never wanted to use its "linking" function for notes since there are so many documents that I just don't want in EndNote but that I still want to tag, write notes in, etc. So it made it a bit complicated to have notes in different places : EndNote + the documents themselves + whatever... I still use it a bit though, but not to link notes or comments. Do you use that strategy a lot?