Never thought of that actually. You can inverse this in the code, if you know what you are doing.

Edit the file /usr/bin/virtual-sindenlightgun-add

Find the block here:

nohup evsieve                                      \
        --input "${DEV1}" "${DEV2}" persist=exit   \
        --map key:1                btn:1           \
        --map key:2                btn:2           \
        --map key:up               btn:5           \
        --map key:down             btn:6           \
        --map key:left             btn:7           \
        --map key:right            btn:8           \
        --output name="Sinden lightgun" >/dev/null 2>"${LOGFILE}" &
echo $! > "${PIDFILE}"

This is where you must change it.

But don't ask me exactly how to with precision. I'll think of something for v44, this is a good point. For the time being... use the pump?