Thanks for finding those articles!
After reading through those, I've spent the last couple hours looking for probiotic supplements along those lines. There are a lot of "beneficial bacteria" products out there, but it seems the vast majority of them are either focused on N-Cycle bacteria or on breaking down waste/mulm in the tank ("Liquid Gravel Vac", etc). However, I have found three that seem like they might be what I'm looking for:
The first is the liquid form of the Aqua MedZyme in your second link:
Aqua MedZyme Liquid
At ~$25 for a 16oz bottle, it sounds pricey, but considering it's supposedly enough to treat 48,000 gallons, I suspect it'll expire long before I run out of it. It's primarily designed for Koi environments, but I'm not aware of any reason why it wouldn't also work for a tropical freshwater tank.
The second is made by the "Dr Tim's" brand, which I believe is fairly reputable for their N-Cycle bacterial products (haven't tried them myself, but I seem to recall seeing that name in a positive light before):
Dr Tim's Aquatics Eco-Balance
A ~$14, 16oz bottle is "only" rated for 480 gallons (quite a difference from the stuff above), but it is specifically marketed for freshwater systems. However, I couldn't find any information the actual bacteria it contains or how many different types there are (according to your links, it's important to have a mix), so I don't know as much about it as I do the first stuff.
The third is this:
Tropical Science Immune-Plus
I've never heard of the brand and it gives no information regarding shelf life, so I suspect that it might not be as high-quality a product as the others. However, it does claim to have 5 different species of bacteria and is supposedly enough for 1700 gallons, so I figured I'd mention it even though I suspect the other two are better options.
I still think the "probiotic" idea is a brilliant strategy going forward (for all of my tanks, not just the quarantine), so I'm definitely going to pursue it. I just need to decide on which product to use for it. Thanks again!