After some discussion, these rules were developed to address what happens when someone tries to slip past defending fighters to gank Pritchard Hood an NPC wizard despite the protection of his allies.
Blocking
If you are part of a contiguous line or dense group of allies, you may freely attempt to block others from passing you; this does not interfere with your melee attack for the turn. Otherwise, you may declare that you are blocking opponents from passing you; this takes the place of your melee attack. (You may not block while engaging in other non-movement actions — spellcasting, missile fire, using wands, etc.)
Passing
Attempting to bypass a blocker or blockers takes the place of your melee attack (or other action) for the turn. Move up to the blocker(s) and roll a six-sided die. Add either your STR bonus if you are trying to push past, or your DEX bonus if you are trying to dodge past.
The blocker may choose to either let you pass and take a free swing as you go by — subject to the usual limit of one free swing per turn — or attempt to block you. For a blocking attempt, the blocker rolls a six-sided die and adds their modifier in that same stat. If the passer's result is higher, (s)he successfully passes the blocker. Otherwise, the passer makes no progress this turn.
The DM determines STR and DEX modifiers for monsters, as well as any situational modifiers. Situational modifiers may make it effectively impossible to pass or block under certain circumstances.
Attempting to move through a line of blockers typically allows two (or more) opponents to block. Make a single passing roll and compare each blocking roll to it. (Multiple passing rolls are required to pass through multiple defensive lines or dense groups.) When attempting to bypass multiple blocking attempts by passing between the guy on the end of the formation and a wall or other barrier, the blocker gets a situational bonus of +2, as it's much easier to block when you can brace yourself against the wall.