Gonna rain on this fan-baiting parade of nostalgia here.
Melissa's currently not under contract to The Flash in any capacity, so the probability of her being available to do this "Hail Mary" stunt for another EP's aging show seems remote, since she's got her YA book series launch happening in October (not to mention her own production development deal with WB-TV). Supergirl wraps filming well before The Flash will even set up before their Season 8 gets underway, and Vancouver still has two-week quarantine restrictions in place due to COVID19. The Supergirl writers room has allegedly already closed down.
Okay, I can hear where you're coming from. But please hear me out.
I'm not saying it's going to happen. But this is definitely not a "hail mary".
1) NOBODY from other shows is under contract with The Flash. Not a single person. But these 5 event episodes are happening, and they are kicking off the season. That is established. All that takes, contract wise, is negotiating a one-off appearance. No matter who they get for these crossovers, that is the type of negotiation that is going to need to happen.
2) Yes, Melissa has her book launch in October, and Vancouver still has two-week quarantine restrictions. BUT, as I previously mentioned, Supergirl wraps in August and Flash is starting production on season 8 in September. Melissa, Chris, and their child already have a place in Vancouver. All it takes is staying for an extra 4-6 weeks at a place they already live. (Side note, there is the possibility that, by then, fully vaccinated people, who can show documentation, may be able to bypass quarantine restriction requirements. This is an ongoing process.)
3) There are 5 "Event Episodes". But there are only 3 Arrowverse shows, other than The Flash, still filming in BC. Legends, Batwoman, and Superman & Lois. The other crossover options are Supergirl, which is based in BC but ending, Black Lightning, which was based in Georgia and has ended, and Stargirl, which is also based in Georgia, and has it's own new season order. It is high probability that, to fill out these "Event Episodes", they will bring in characters from at least 2 out of these 3 properties. So no matter what, SOMEBODY is going to have to come to BC from the outside and quarantine. But as previously explained, it would not be as big a hurdle for Melissa, who is already here, and already has housing. No matter who comes to do the crossover from the States, they will still likely have to arrive in late August to start their quarantines.
4) Melissa's book launch isn't until October 19th. If The Flash starts filming these "Event Episodes" in September, that's plenty of time to get her back to the States for her launch. And that's with the extremely generous assumption that she needs to physically be there. Maybe she'll do physical book tours, maybe she wont. But with how private she is, and the ongoing caution regarding COVID, despite vaccine rollout, it's actually probable they may not do any sort of physical touring book launch with Melissa and her sister.
5) Melissa's production company has a contract with WB. While her work on Supergirl pre-dates that contract and does not have ties to it, agreeing to do a one-episode crossover would help continue to foster her good relations with Warner.
Again, I'm not saying Melissa will participate in these event episodes. I just wanted to illustrate why I wouldn't classify it as a "hail mary". Honestly, of all the crossover options, The Flash, as a show, has the most history with Supergirl, and narratively, makes much more sense than crossovers with Legends, Batwoman, Superman & Lois, or Stargirl. Am I being hopeful? Yes, absolutely. Largely because the COVID restrictions, this past season, have robbed us of one last crossover involving Supergirl. I can acknowledge that I am being a bit of an optimist with this.
If I'm being totally honest, I personally wish that one Event Episode would involve both Supergirl and Superman, to give them one last interaction, as well, before Melissa hangs up the cape. But I'll admit that is wishful thinking.