Hmm. Thinking about this, it doesn't seem possible. What happens to constellation placement, for example, if there's a new core star system? That'll affect the valid locations for a constellation and have knock-on effects that cascade through everything. Or, if a mod such as Unknown Skies adds new planet types etc - having additional options to choose from will influence the selection even if the RNG seed is the same. There are likely other things I'm not thinking about right now that would also muck it up.
I hear you as far as seeds being pretty useless when mods are involved, though...