Is it straightforward to set up a seed node that's simultaneously available on multiple networks? For example tor, i2p, clearnet, and maybe even yggdrasil all at the same time. I ask because I recall (not git related) projects in the past that did not do well when configured in such a way.

Tor only (SOCKS) or Tor and clearnet is easy. Yggdrasil only is also easy (just restrict access to your tun). Yggdrasil and clearnet requires you to ensure that outgoing traffic takes the correct interface, this also applies if you want to combine with Tor. I don't know about I2P.