Just got off the phone with a PA brewer (who prefers to remain anonymous...and as long as the BLCE keeps that up, so will I), who told me they'd talked to the brand registration folks at the PLCB and had this to add to the discussion. Vintage beers -- older beers that are no longer brewed, or beers from defunct breweries like Heavyweight or King & Barnes -- that have already been purchased by a retailer are not in danger of causing a citation for unregistered beers. Once beer is purchased by the retailer, assuming it was registered when purchased, it is "grandfathered in," and the retailer doesn't have to worry. So if you're a bar owner or a distributor or a deli, and you've got beers you're aging? Don't worry about it.
Well, not exactly. Note that wholesalers are not afforded this protection, and there is nothing to keep the BLCE from checking their list, not finding those aging beers, and confiscating them. But you can't be fined or suspended for having them, and you can file paperwork to get them back.
I'm not sure just how good I feel about this.