Well now that I am thinking about it more, and this is debatable, but your first idea is probably the best one. If you put an Oldblood in a unit with a Scar-Vet BSB using the Standard of Discipline, then technically the whole unit is Ld 9. So long as your Oldblood doesn't leave and the BSB doesn't die. And they would have to be with regular saurus to avoid making the Slann Ld 10. The rules for the standard say that the unit cannot use the Inspiring Presence rule, and since the saurus are all Ld 9 it wouldn't matter anyway. You will have to clear that with the tourney organizer though.
Man, I don't know if there's a way past this. You could pull the Wizarding Hat on an Oldblood, but that's only a 1/8 shot of getting Metal. Not very good odds. And that's just a little cheaper than a vanilla Slann. If only you could put the Sacred Steg Helm on a saurus.