I use PowerAuras on my main to monitor the rest of the team for 'special' buffs/procs etc...

http://www.wowinterface.com/download...asClassic.html

I have one checking partymembers for the "Hot Streak" buff, so I know my mage can fire off an insta-pyro.
When it spots a Hot Streak buff on any party member (which will be my mage) it pops up some text on my (tank) screen to prompt me to press the key for my stopcasting/pyro macro.

Similarly, if my mage is in arcane spec, I have another PowerAura monitoring for "Missile Barrage" buff, so I can trigger off a stopcasting/ArcaneMissiles macro.

If "Lock and Load" is a buff the hunter receives then you could similarly monitor the party for the buff and pop up a prompt on your main.