UKC

I climbed a route not listed on any crag site/Rockfax app while in the Fruitbat Level of Australia quarry in Wales this weekend.

How would I go about adding it to the guides so others can climb?

There is a button on the crag page on desktop.

In Scotland you would also tell the SMC, not sure if there is something similar in Wales (or the particular area).

If it's already bolted I would bet my house on it already having being climbed

The Rockfax guide is not definitive. I very much doubt you did anything new there, unless it was complete choss. Sorry. 

Lookup the crag moderator and ask them, they might know about it already

Well sure, if it was bolted I would assume it had been climbed! It wasn't, it was a chossy crack.

I'll give you a less condescending reply, because contrary to views given here there are still plenty of new routes being done on slate (even if they're not necessarily very good).

Wales is sadly lacking any centralised resource like Scotland has for recording new routes. You could just record it on UKC by clicking 'add missing climb' on the crag page. You could contact Rockfax or Mark Reeves personally, as they are the publishers of the most current guidebook, and the route might be considered for future editions.

I believe there is also a new routes book recently installed in V12 in Llanberis, but I don't know if that's taken off or is even widely known about yet.

There is also a new routes section on the climbers club website. 

Not over obvious though, I found it by menu, publications, then submit a new route or look at archive for routes presumably not yet in or missed out of  a cc guide.

As Andy has stated. The easiest way is via UKC log books. Click on the add new climb at the bottom of the page, add to buttress section then name, grade and brief description. Pop in your name and date it as climbed and then the crag moderator will relocate it to the relevant position. Sometimes people will retro claim a first ascent because it hadn't been recorded at the time but that’s just a fact of life. Hopefully it’s new and others will then get the opportunity to repeat and enjoy.

I usually put it on UKC + Climbers Club for north Wales 

Not if immediate use, but the CC is in the process of developing a new website which will include a new routes database similar to the SMC's.

I think though that it only includes areas covered by CC guidebooks - I can't see any slate on there, unless I'm missing it?

I have submitted a few things to that myself, but it doesn't seem to be universally used.

You're not missing anything, at the moment the new routes section on the CC site is rubbish. However we await great things in the near future!