I just replaced the shock cord in my boom a couple weeks ago. It is 7' long. There are 2 aluminum blocks inside the boom the out haul cleat bolts into, fore & aft. Each has two holes in it, port & starboard. The shock cord routes through both starboard holes, through the hole in the end of the traveler car, and back through both holes in the port side. The ends are knotted off.
It might be possible to route it without removing the end cap, but I wouldn't recommend it. The slot along the top side of the boom is really thin. In my opinion difficult sounds like an understatement. Besides, with the right equipment removing the end cap isn't difficult at all.
The rivets are stainless 6/6 size. You can get them easily at your Hobie dealer or search online and find many sources. Their diameter is 3/16". There are different recommendations for drilling out the rivets.
1. Use an over-sized bit, like 1/2", to drill off the head. Then use a punch to push the rivet through.
2. Use the same diameter bit to drill the rivet all the way out.
With the second option you run more risk of enlarging the holes so a new rivet may not hold as well, or at all, so the first is more recommended. However I did find that due to the shape of the casting sometimes there isn't anywhere for the rivet to go. If you find you can't get the rivet to punch through you may have to drill it all the way out, but be extremely careful here.
Installing new rivets is pretty simple too. They are stainless, so they are harder to pull than a standard rivet. I recommend a heavy duty bolt cutter mechanism style puller. You can get them for ≈$20 at Harbor Freight or Northern Tool. I've heard others complain about how difficult pulling the stainless rivets can be, but I had no problem at all with one of these. Be sure to wear eye protection though.
Also, while you have the cap off, you may also want to replace your outhaul line too. 7' x 3/16". It ties to the end cap, around the outhaul car pully (in the bottom, out the top), around the end cap pully (in the bottom, out the top), and to the cleat.
Finally, you will also want to inspect your outhaul car as well and replace any parts that may be worn out.