I just ran a series of experiments on the availability of Gold Peaches. Here are the results:

I took a monster that I had used as a "dummy monster" in DP's method, which had high stats, but had never gone to an official battle as my test subject. (Durahan/Joker, in case that is of interest.) When I started the experiment, he was 4y 5mo, and had all stats at 999 since he was a leftover from an old gameshark experiment. There was no gameshark use during the experiment at all.

First series of experiments were run by saving my game in week 3 of September, then battling the D level tourney to get to C rank and going on the expedition. I only checked the tusk tower, and went home each time. At C rank, I always found 2 disc chips, and either 2 bags of soil, or a bag of soil and a half-eaten. I did this 7 times.

Then, I waited til the next year, keeping the monster in the freezer, at C rank while I raised another monster.

The next series of experiments was interesting. Once again, I saved in week 3 of September. Two times I rested in week4 and sent him off at C rank. Same result: 2 disc chips, plus 2 bags of soil found on both trips.

Then I re-set and sent him into the B-level official tourney, raising him to A rank, and went on the expedition. I did this five times. Each time he returned with one gold peach and 5 disc chips. (Remember, in this last series, he has the same age each time and the exact same stats.)

Conclusions:

These experiments may have been too few in number to have proven anything, but further tests by others are welcomed.

It appears that rank is a factor in finding peaches. I found peaches 100% of the time with an A rank monster. (Did not test B rank at all.)

There may be other factors, like stat levels, which have not been fully tested, affecting the outcome.

There is still a great deal of randomness associated with the placement of peaches, too. So, be patient!

Some genral Peach tips:

The gold peach sets the monster's internal clock back 50 weeks, silver 25 weeks. You won't see any indication of this in the game.

You can only feed one gold and one silver peach per monster. But, unlike MR1, you can send a monster to find as many peaches as you like.

Use your peach at about 1 year after the start of "prime". If you can't figure out when that is, look in the FAQ section for a thread called "Life Stages the Real Scoop".

Don't give a peach after you have gotten the old age warning, it will not add a year once that milestone has been reached.

Yes, I also have found this to be true. But I still cannot find it 100% of the time at S or A.