Using iptables to set ToS
X-Lite and Twinkle don't have a way to set ToS. So let's get iptables to do it:
iptables -A POSTROUTING -t mangle -p udp -m udp --sport 8000:8009 \
-j DSCP --set-dscp 0x2e
That sets the DSCP for UDP packets with a source port of 8000-8009 to 0x2e, which is the Expedited Forwarding PHB. You'll need to pick a range that works for you. Both X-Lite and Twinkle use 8000 as a default, so that's what this rule says.