My front passenger brake seems to make a grinding noise, felt slightly through the pedal.

It happens when either I press the brakes fairly hard ie, lights turned amber and need to slow down fast, or if they are being used a lot (ie, on a longish trip through twisty roads).

I don't drive fast/hard and use the brakes gently on a daily basis.

The car has just passed 200,000km.
Pads look like they could use a change, seems by eye like about 8mm or so material left.

Just wondering if this is a symptom of any known issue like bearings, low pad material, damaged rotor etc etc?

i would be taking the pads out and inspecting them your self they dont always wear even and if they are worn more then the thickness of the backing pad then change them out. Go to super cheap they charge $45 and they are actually Bendix pads

My front passenger brake seems to make a grinding noise, felt slightly through the pedal.

Not good. Pull pads, check disc surface. Looking at the outside pad tells you nothing about the inner pad. Grind + vibration = pad backing rubbing on the disc.

It doesn't happen always, just at strong brake levels or during long trips.
It's been doing so for quite a while...
Could it be the wheel bearing?

Dont take any chances, if you know what you are doing, get them off and have a look. Or get someone who knows to take a look!
Tyres wheels Brakes and suspension should always be a high priority for your safety... Just sayin'

Just check both pads and both sides of your disc. Are the discs smooth and clean over their entire contact area? Are the caliper slides lubed and able to slide smoothly?

Usually a rear brake problem will be felt through the pedal. A front brake problem will be felt through the steering wheel.

Maybe pull all 4 wheels off and check the lot.

It doesn't happen always, just at strong brake levels or during long trips.
It's been doing so for quite a while...
Could it be the wheel bearing?

It's all very fine posting the minute details of the problem, but until someone gets active and does some exploratory investigation, you and everyone else is just guessing.

"exploratory investigation" = pull the offending wheel off, and examine both sides of the disc, and both pads.
My guess is that the inner pad is worn down to the backing plate and the disc is ruined, as you say, it has been going on for quite a while.

Don't drive the car until you have identified the problem and assessed implications for your safety.

/start lecture

Brakes should never be neglected. At the first sign of a problem, such as a strange noise, car pulling one way when braking, or one wheel locking up on a wet road, the brakes should be looked at.
Regular inspection should include looking at the thickness of pads, leaks, callipers sliding freely on the guide pins, and the condition of hoses.

/end lecture.

I will be looking at the rear brakes on my TL wagon this coming week. They are working fine, but I haven't looked at them for 30,000 km. New pads will go in if they are anywhere near the limit. Just to be sure. Preventative maintenance. I've already done the front, and both ends of my TJ.

What is the limit for brake pads?
And, if the rear pad is worn down but the front is fine, does this signify something that needs replacing/fixing?

Rear pads don't wear no where near as much the fronts. When was the last time the rears were checked/changed?
The noise and vibration you felt from the front could very well be the rears also.

What is the limit for brake pads?
And, if the rear pad is worn down but the front is fine, does this signify something that needs replacing/fixing?

You mean the inner and outer pads on the one disc?
2 mm is the recommended minimum thickness for the pad material itself. But you would have to take them out and look for the thinnest part. I've seen pads with 2 mm or more at the upper edge, while the material at the bottom edge is completely gone and giving that lovely metal to metal grinding noise when you brake.
If the inner and outer pads wear at different rates, it is usually because the sliding pins are clogged up and aren't free to slide.
You can also get different rates of pad wear on the left and right front discs, this is due to one piston (inside brake calliper) being sticky and not working as hard as the other side when you brake.

EDIT: If you don't feel competent to dismantle and check/replace things yourself, take the car to a brake shop and tell them the symptoms.

Thanks Max,
Yes I did mean inner/outer brake pad.

I think I will likely just get new rotors and pads and have them fitted