I believe if any pieces do get re-circulated into the engine they must do so through the EGR system? (someone correct me if I'm wrong) At least on the 3.5VQ, it's just as common to get EGR valve codes because little bits of catalytic converter are stuck up there as a actual 02/cat code. So you could pull your EGR valve and see if there's any random particles in there. It's possible that if there is, you can pull them out and save yourself any future damage.
Compression test is the best way to check out the health of your piston rings. The gauges are cheap (~$50) so I would suggest buying one yourself and learning how to use it. It's a great diagnostic tool. Just remember to always do a compression test with the engine warm. (all parts heated up and fully expanded/seated) Doing a compression test on a cold engine can sometimes yield some scary results and lead you to spend money and perform repairs that may not be necessary.
A cylinder leak down tester is a good tool to have as well. You rotate the crankshaft until the valves are closed on each cylinder (one at a time) and then you pressurize the cylinder with compressed air. If the gauge on the tester does not hold, you are leaking that air out of the engine somewhere. If it's escaping past the piston rings, you'll usually find that the air is coming out through the dipstick tube. If a intake valve/seat is bad, the air will come out somewhere like the intake manifold. If a bad exhaust valve/seat, it'll come out the tailpipe. A piece of tissue paper or equivalent is good to use to help detect any air flow because sometimes the amount can be small and not detectable by "feel".
2005 Altima SER