You could possibly get the info you need from d-bus.
For instance - if the job is already running (which it should be to trigger the pre-capture script), then the Exposure time could be determined from D-bus using the Kstars/Ekos/Capture methods "getActiveJobID" and "getJobExposureDuration".
However, if you need to know this before the capture is started - it won't work, because getJobExposureDuration needs an active Job ID to return the exposure value.

I tested this with python and pydbus and it works. (was looking for an alternate to monitoring RTS for something I'm working on and came across this thread)

from pydbus import SessionBus

# Connect to d-bus session bus
bus = SessionBus()

# Create an object for kstars/capture - allows access to properties and methods.
busObject = bus.get(
    "org.kde.kstars", # Bus name
    "/KStars/Ekos/Capture" # Object path
)

# Introspection returns an XML document containing information
# about the methods supported by a d-bus object 
#print("Introspection data:\n")
#print(busObject.Introspect())

#  get active job ID
id = busObject.getActiveJobID()
# print exposure value for active Job
print(busObject.getJobExposureDuration(id))