I now think oscam does not have the capability to show which key is currently in use (i.e. the ECM).
From the page EnoSat points out above:
ecmfmt = format
define ECM log format, default:c&p/i/s/l:h
possible variables:
c = CAID
d = PID
e = CSP hash
g = ID of origin gbox peer
h = checksum
i = Channel ID
j = distance of gbox and CCcam hops
l = length
o = ONID
p = provider ID
s = service ID
t = tier ID , this ID will be replaced with the tier-description from oscam.tiers (if found)
w = CW
y = payload (short, 3 bytes)
Y = payload (long, 6 bytes)
I don't think any of that data includes the key number.