So, I spent some time in Stormwind with my new best friends - the targetting dummies. Here are my results:

Attempt:
/cast Lava Burst
/castsequence Chain Lightning, Lightning Bolt, Lightning Bolt

Result:
Lava burst would cast, castsequence would never fire - not CL, not LB, lava burst would recast when cooled down.

Attempt:
/cast Lava Burst
/castrandom Chain Lightning, Lightning Bolt

Result:
Lava burst would cast, castrandom would never fire - not CL, not LB, lava burst would recast when cooled down.

Attempt:
/castrandom Lava Burst,
/castsequence Chain Lightning, Lightning Bolt, Lightning Bolt

Result:
Lava burst would fire once, then the castsequence would continue until OOM.

Attempt:
/castrandom Lava Burst, Lava Burst,
/castsequence Chain Lightning, Lightning Bolt, Lightning Bolt

Result:
Lava burst would fire once, then the castsequence would continue until OOM.

Attempt:
/castrandom Lava Burst,
/castrandom Chain Lightning, Lightning Bolt

Result: Lava burst would fire once, then the castrandom would continue until OOM.

Attempt:
/castrandom Chain Lightning, Lightning Bolt
/cast Lava Burst

Result: Castrandom would work fine, lava burst would never be cast.

Attempt:
/castsequence Chain Lightning, Lightning Bolt
/cast Lava Burst

Result: Castrandom would work fine, Lava burst would be cast about a third of the time (same as /castrandom CL, LB, LaBu)

Attempt:
/castsequence Chain Lightning, Lightning Bolt
/cast Lava Burst

Result: CL would cast, then LB, then LaBu, then CL, then LB, then wait for CL CD, then CL, etc

Attempt:
/castsequence Flame Shock, Lava Burst, Lava Burst
/cast Lightning Bolt

Result: FS cast, LaBu cast, LaBu cast, Lightning Bolt cast

Final result: No way to prioritize Lava burst using just one macro, I've ended up using one button to apply flame shock and a /castsequence lava burst, chain lightning, lightning bolt, lightning bolt, lightning bolt, lightning bolt solution

The final result has some downsides:

1) Chain lightning doesn't fire as soon as it's up (this will stop being a problem once LB becomes higher DPS for single targets)
2) Lava burst doesn't fire as soon as it's up if heroism isn't active
3) Reapplying FS for trash mobs is annoying

So I ended up making two versions - one for trash and one for bosses.

Boss version:

Button 1: Cast flame shock
Button 2: Castseq: Lava burst, Chain Lightning, Lightning bolt x 4

Trash version:
Button 1: -
Button 2: Castseq: Flame shock, lava burst, Chain Lightning, Lightning bolt x 3

I might need to add a 3rd lightning bolt once I get more haste.

One interesting thing to note is that the trash version (LB x 3) with manual FB application (when about to run out) was in terms of DPS not a significant improvement over just reapplying it before casting LB, which I don't really understand tbh.