Here is part of your answer.
This link shows the opening times for the left luggage room at Meiringen:
www.sbb.ch/content/int ernet/sbb/en/bahnhof-services/bahnhoefe/bah nhof-finden/bahnhof.8308.ht ml
If there are lockers, they are usually listed under this same heading as the left luggage offices ( sometimes one is under services, the other is under equipment) on the SBB website. I checked Luzern for a test, and lockers are described
I do not find lockers mentioned at Meiringen. I am surprised. I did not find them mentioned at Interlaken Ost, either. That surely is not correct, in my opinion. Perhaps they have not finished the new format for the SBB site, or, possibly, I'm just plain out of touch. Below, I suggest some ways to find out.
Also, in the version of the web site in place until a month or so ago, you could find prices for lockers and left luggage. I don't know how to find that information in the new format.
I'd guess between 5 and 10 CHF per piece to leave luggage at the Left Luggage Room for a day or a part of a day. Does not matter if you have a pass or not - price is the same, but you might be asked to show a ticket or pass. That is necessary when you ship your luggage ahead; however, they may not care if you are leaving your luggage for a few hours or a day.
There is a phone number on that link above for Meiringen station. Or, you could contact the Tourist Info Office at Meiringen:
Regarding your train options - for trains on the Golden Pass route as on any other Swiss train, your ticket is good for a day, does not require reservations, and thus you can get off a train along the route, look around, and catch a later train to continue.
If you reserve special seats on the two kinds of panorama cars available on the Golden Pass Route, that freedom disappears. However, from Luzern to Interlaken ( called the Luzern -Interlaken Express) , the special trains where you can sit in the front of the first car in a big window area are not available. They are only available after you get to Zweisimmen. And, the train from Meiringen to Interlaken is only a very short ride. 5 minutes or so. So, I would suggest that simply catch the next train after you have finished looking around.
You also could consider the cost and timing issues if you go directly to Interlaken, check in, leave your luggage at your hotel, and take the short ride back to Meiringen.