On an onside kick, what happens when a return man signals fair catch and is obstructed from reaching the ball?

Question

Team A is still down by two points with less than one minute remaining in the game and they are kicking off to team B after just scoring a touchdown. Team B realizes they are going to try an onside kick and at last minute brings their return man up right behind their first line of defense-men. As team A is kicking the ball, the return man raises his hand for a fair catch in accordance with Rule 10. As the ball is kicked sideways and the return man runs toward it, he is blocked both by his own men and players from the other team from reaching the ball.

And the question: Would this be legal under the rules? Would the ref call some type of foul on either team? Is the return man entitled to a clear path to the ball under this scenario? And if not, please cite the rule.

Update: I'm intrigued by Michael's response; but not yet quite satisfied. Let's add another facet to the scenario. Let's say the wind is blowing hard when Team A is kicking off and they can't keep the ball on the tee and they opt to drop kick the ball as provided in the rules. So has the ball touched the ground and now the kick off receiver can't call a fair catch no matter where he is standing? And if that is the case; wouldn't be better for every team to drop kick the ball so they could run down and tackle the opposing return man?

Answer 1

Let me quote from the rule you have cited. Rule 10, Section 2, Article 1 of the NFL Rules:

A Fair Catch is an unhindered catch of an airborne scrimmage kick that has crossed the line of scrimmage, or of an airborne free kick, by a player of the receiving team who has given a valid fair catch signal.

The key word is "airborne". Once the kick has bounced, it is no longer considered airborne. If the onside attempt is a "pooch" (short high kick), then even if no one signals for a fair catch the kicking team would be penalized if they interfered. By Section 1, Article 1, item 2:

A receiver who is moving toward a kicked ball that is in flight has the right of way. If opponents obstruct his path to the ball, or cause a passive player of either team to obstruct his path, it is interference [...]

Again, note the qualification "in flight".

This is why most onside kickers attempt to either roll the ball along the ground or kick it down hard so that it bounces high. A high bouncing kick is ideal, since the bouncing is likely to slow it down enough that the kicking team can get into position and block the receivers.

---

Note that "airborne" and "in flight" are not explicitly clarified anywhere in the NFL rules that I can find. A high bouncing kick could be construed as airborne by the dictionary definition; in fact, the NCAA has recently made exactly this decision:

Also, receiving teams can call for a fair catch on onsides kicks that are driven directly into the ground but that don’t bounce a second time.

The NFL has no such rule, though.

---

The NFL Official Casebook clarifies the issue.

A.R. 10.8 INVALID SIGNAL—KICKOFF HITS GROUND
On an onside kick from the A35, A1 “tops” the ball causing it to go down into the ground and then bounce high in the air to the A46 where it comes down to waiting B1. B1 signals for a fair catch, but just before B1 catches the ball, A2 blocks him aside and the ball hits the ground and is recovered by B3 at the A45.
Ruling: B’s ball, first-and-10 on B49. Invalid fair-catch signal as the ball had hit the ground, so the action by A2 was legal. Spot enforcement.

(hat tip wax eagle)