No, he can't since he must flank.
Unfortunately the reason is implied in the words used to define the assault leader feat
Once per day, when the rogue misses with an attack on a flanked opponent, she can designate a single ally who is also flanking the target that her attack missed.
As we can see the feat writes clearly "attack on a flanked opponent" and a bit later says "an ally who is also flanking". So yes...you must flank the enemy to use this feat but the ally you are designating doesn't have to be in a flanking position with you.
Let's run an example:
an enemy is flanked on every side (north, south, east, west) and the rogue is on the east side of the enemy. Even if the rogue is flanking with the ally on the west side of the enemy he can designate every other ally since all of them are flanking the same enemy the rogue is flanking.
I hope it is clear enough.
Now, talking about flanking with a ranged attack the answer is again a no since flanking unfortunately says:
When making a melee attack, you get a +2 flanking bonus if your opponent is threatened by another enemy character or creature on its opposite border or opposite corner.
So normally you can't flank with a ranged weapon but there is this feat:
While wielding a ranged weapon with which you have Weapon Focus, you threaten squares within 5 feet of you. You can make attacks of opportunity with that ranged weapon.
Here is a bit tricky. Having this feat you can provide flanking bonus to an ally with a melee weapon but since you have not a melee weapon you are not getting the same bonus when attacking.
Now if you don't get the bonus to attack is the enemy flanked by you? RAW it seems not but i think there is a lot of table variance about this topic and therefore my advice is to talk to your GM about this.