how would I go about. . .

[rrgg]

Allowing javelin armed troops to move while shooting, the way missile cavalry can.

I'm not too worried about animations looking right, i just want fire while moving.

[crzyrndm]

Missile cavalry use the attribute mounted_fire_move to achieve this. You could try adding it to missile infantry, although its doubtful it will work as missile cavalry lose the attribute when dismounted