Frankly, I'd prefer having draftees doing all the scut work: slopping out latrines, etc. Using them as cannon fodder is counter productive.
Personally, all I'd do is say that only those who've done voluntary active duty can attain political office and qualify for twice as much unemployment or welfare as regular citizens.
Your Heinlein is showing. 
Why not go all the way and simply deny them citizenship unless they opt for military service?
Optimally, any libertarian society, living in a world still dominated by statist tyranny, would need some mechanism both to plan and provide for, and be capable of, rapidly flipping from a highly decentralized libertarian society to a highly centralized military one when it is needed. A major problem is that those who never experience the horrors to which statist opponents of liberty are willing to go to, tend to go into denial and avoidance to actively prevent such a society from making the necessary flip when it is needed.
Those who have never served a day in the military have, in my opinion, some of the most loony, idiotic, foolish, ill considered, naive opinions of other nations, leaders, and movements. While I can always find some veterans that disagree with me on foreign affairs, I must say they almost always have well informed reasons for their opinions. Finding such well informed individuals among the non-veteran population is a few and far between proposition.