I'm going to guess this is unavailable in the current game.
Prior to Burning Crusade (broke, with 2.x patch), there were addons (my rogue had one) which would choose your most beneficial move when you clicked the button.
Blizzard has steadily moved towards us choosing (whether at the time of the click, or beforehand when constructing the macro) our own move.

A macro set up with a fall-through used to work like this, last expansion.

They might still work this way; doesn't hurt to test.
#show
/castsequence Lightning Bolt, Lightning Bolt
/cast Lava Burst

I know a similar macro still works for a Holy Paladin.
#show
/castsequence Flash of Light, Flash of Light
/cast Holy Shock

It's not so much that Lava Burst or Holy Shock are unclickable.
Rather, if you're spamming faster than 2 clicks a second...
The fall-through fires off when available.