If you pull out Flame Shock and use that macro as it's own cast button {manual cast} it should work that way. That is, the first time you press that macro, it'll put up Flame Shock as long as you aren't on the global shock cooldown, and any time after that it'll do Lava Burst until you change targets. However, there's always a chance that it won't work flawlessly, which is true of any macros we use, and it'll try to cast Lava Burst before Flame Shock on occasion. Or there's always the chance that Flame Shock could wear off, but that can be remedied by adding additional reset conditions, like reset=target/alt so that you can use a modifier to manually reset the /castsequence part and reapply Flame Shock.

You could also just swap the macro directly into your spam sequence to replace the Flame Shock that's already there, because the null in the macro won't reapply Flame Shock until you change targets or manually reset it if you add that condition, so it won't keep locking out your Earth Shocks. I use that method with my Death Knight to apply diseases once before anything else happens. Though I can't speak to the effectiveness of that setup for Elemental Shamans in particular since I don't run one and I'm not familiar with their optimal priority setup.