Any camera that does record at 9fps should also record the subsecond  
I agree, especially since there is a 
standard Exif tag for sub-seconds.  Well 
kinda: it is a "string" but the Exif spec isn't fully clear about what the values mean or whether they are zero padded for sorting purposes.  But for Nikon cameras (e.g. D300) this is a two digit (approx 1/100 second) zero padded value so it works nicely.  Last time I checked Nikon wrote this information but Canon didn't.
Kirk is right - no need to add metadata to your photo, just use {dats}{tmsl} to sort if you have subseconds (e.g. Nikon), or try {dats}{tims}{fnm4} for the custom sort if you don't have subseconds.  This sorts by date/time first, then by four digit frame number which you should have if you ingested using PM, even if you renamed your photos on ingest.  (Caveat - the latter won't work when your frame number advances from 9999 to 0000 in the same second 

).
--dennis