Thanks all!
While the docs do say you can specify multiple ip= parameters, booting the installer with one ip= parameter for IPv4 and one ip= parameter for IPv6 on the *same* interface (also adding the required bootdev/rd.neednet/etc parameteres described in the docs as well as without them) does not seem to work with these installer images:
Fedora <32 EL7-9
It *does* work with Fedora 32 through 40.
For Fedora 32 through 40 I can just supply the two ip= parameters (one for v4 and one for v6) and the ifname parameter, and that works (without rd.neednet/bootdev/etc).
Is there some parameter I'm missing? I can supply the kexec line, for example, but it's difficult to give more debugging information than that as the console halts/does not print anything when booting into these kernels with the two ip= parameters and just hangs.