Don’t worry about it!
I’m disinclined to use any fly-specific features (hopefully for obvious reasons).
Since Fly assigns public IPs (albeit anycast) per-app and not per-instance (like other VPS out there), DNS-foo wouldn’t be of help, either. (see edit below) You are relegated to use N apps, I guess.
Also, if you do change your mind, a better Fly-specific solution (if you have a say over the code that runs on the clients) for your case would be to use the Fly-Prefer-Region header instead (1, 2).
Edit: Looks like one could assign regional IPv4s. Along with some DNS-foo (1), one could possibly redirect traffic to a specific Fly region. I’d guess the usual $2/mo charges apply for these additional (regional) public IPv4s.
# supported regions: iad, lhr, nrt, scl, sea, sin, syd
fly ips allocate-v4 --region iad
Ref: Unable to run allocate-v4 with the --region flag - #3 by kurt