The two weapon rules cover weapon and shield fine, Terry. You're only penalised for attacking with both weapons; fighting with a main weapon and parrying with the off hand one is normal, and shields get their parry bonus.
Also an SCA veteran. I am a little wary of saying outright that you can parry an arrow with a sheild, given that for safety purposes SCA combat arrows are low power and a bit slower because of the rubber tips, but as a general rule you only need a fraction of a second to react to an archer that you already are aware of and can see is getting ready to draw. If they are sufficiently far away, even with combat bows shooting pointy arrows, it is practical to get your shield where it needs to be. Call it a Block if you like, but it's an active defense, so using Melee to parry feels better than a flat -2 cover defense. If the missile is something thrown, it's even more reasonable; maybe apply both? But if you are not actively defending against a specific attacker, such as in a shield wall, it's definitely a Cover situation.
In game terms, shield parrying of arrows doesn't exist, so it's only an idea if you like that idea. I'm certainly not going to crusade for it. -2 cover mod works, and in a normal Traveller environment you rarely have shields defending against bows, so it's a minor edge case (riot shields vs thrown objects could well come up at some point).