Woot! So it turns out I can add life tap to my pet attack macro.
/target [help] targettarget
/petattack [target=pettarget,noexists] target
/cast [nochanneling] life tap(rank 1)
If my target is friendly to me (ie, the tank) use my target’s target and ..
If I’m not currently channeling another spell, Life tap (rank 1) (because I have the Life Tap glyph) and …
…If I have a target and my pet currently isn’t attacking anything, send my pet to attack my target.
…If I have a target and my pet -is- attacking something, call my pet back to my side.
Paraphrased: hit once to have pet attack, hit again to call pet back to you.
[Edited to add: minor tweak made to above so that the Life Tap doesn't interrupte me if I'm doing a channeled spell]
I’ve update this on my warlock macro page.