Sam,
I run a Lotus Notes shop, so am not the best to advise on M$ Mail. Now this is going back a ways, to WFW 3.11 actually, but back then it was stored in an INI.
You have to think like the program. It too has to find the file. So, if it is in a unique key, then somewhere there has to be a value that IDs that key. A bit of reverse sleuthing. Search for the value holding the path. If in unique key, search for the value pointing to the key.
Alternately, you could recurse the reg keys and do your own search. Do a search on this board on the words "PST location". Here's one of the hits:
KIXTART-SCRIPT: collect information about your Outlook settings
Maybe someone with current Outlook exp can step in here.
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Give a man a fish and he will be back for more. Slap him with a fish and he will go away forever.