I currently use macros when healing, so that whenever I start casting a heal, I automatically switch to the appropriate totem.

Here's what my macros look like (using Clique):

/cast [target=mouseover] Healing Wave(Rank 1)
/equip Totem of the Maelstrom

Simple enough, but it's bringing up an unwelcome problem. If I try to heal a target that I cannot (they are out of range or los), it will swap to the totem of that heal anyways, placing me on global cooldown.

So the question is, can I write macros that only switch totems if I begin to heal a valid target?

Can you pass target parameters to the /equip function? Like: /equip [target=friendly] ...

well, target=friendly wouldn't work, because I don't target who I heal.

I don't think there's any way to make this work like you want to; macros simply can't check for LOS or Range, so there's no way of preventing the macro from equipping the totem. The only thing you could try is to change your /equip to "/equip [channeling:Healing Wave] Totem of the Maelstrom", but I'm unsure whether it would ever process (It depends on the way the client/server fires events).

I'm also not even sure whether channeling as a conditional applies to normal spell casts, or if it only applies to channeled spells like the name implies.

I thought there was an equip cooldown when switching relics?

I may be mistaken, but I don't think this would work at all due to that cooldown.

I remember running karazhan and attempting to do this via Itemrack script commands. It worked horrible. Better off judging what heal your going to use the most and then putting that totem on.