After my searches being fruitless for quite a while now recently I found a single piece of information that helped me:
Flow rate through Schrader valve with 8bar supply pressure= 157L/min (2.6L/s)
Initially used on a forum for calculating something or another to do with homemade scuba gear,* or something of the sorts, the test compromised of putting constant pressure through a fully opened Schrader valve and timing how long it took to fill a 20l bag.
By converting the units to cubic feet per hour, and thus by using using the equation "Cv = q [SG (T + 460)]1/2/ 660 pi" (thank you engineers toolbox) I got the result:
Which should make the Cv of a schrader valve out at around: 0.12
As soon as I can get my own equipment to test the valve coefficient accurately and so get a more reliable figure for it I will, but until then this makes a good alternative to simply guessing a Schraders valve coefficient, and would be a better than guestimate figure to put into GGDT or the equivalent for people attempting to simulate a Schrader valve in their design.
Alternately if anyone has a good alternate value (and tells me I am massively wrong) I would love to hear it, as searches have returned nothing, even from schraders own site, and the data I have used is less than reliable for the function I used it for.
*There were other results that were posted, but the others had different variations in connection and had some manner of restriction between the valve and the tank so I decided to disregard these