Well, both Alice and Yuuka did recognize Reimu and/or Marisa (and vice versa to varying degrees) -- Yuuka especially, commenting on how Marisa's grown up and such. It's just all indirect, with no mention of specifically what their history is. For all we know, their PC-98 backgrounds are completely gone and they know the mains for some other reason. Still, the reference is there.
Following their pattern, I think it's possible for more PC-98 characters to show up, but there can't be any consequence to it. Someone like Mima is less likely to return because she was so important before; having her show up now would raise awkward questions. (I believe ZUN talked about this once.) At the same time they'd have to be interesting enough that ZUN would want to bring them back rather than design someone new.