Can Opus natively extract multi-volume RAR archives? I tried, selecting the first part (*.r00) but was met with the error seen below. I had to resort to using WinRAR to do this, which I do have installed but would like to use as little as possible.
Yes, it can.
Are you sure the .r00 was the first part, and there wasn't a .rar file lower down the list?
WinRAR will allow you to start extracting from the middle of the archive, skipping any files that started before the part you opened. Opus won't let you do that (as it's almost always an accident, and in the rare times you might want it you can use another tool) and will show that error message when you try to.
Ah yes, there is indeed a .rar file which I neglected to see (was the last in the list), which does allow me to extract the volume. Not only that, it's a hell of a lot quicker than WinRAR too (at least three times quicker).
So, essentially the only reason to have WinRAR installed now would be to create RAR files?
Not sure why Opus would be so much faster than WinRAR there, but can't complain either. 
You'll need WinRAR to create and modify RAR archives, even in Opus. (Opus has built-in un-RAR support but for anything that modifies RARs it needs to call WinRAR behind the scenes.) Aside from that, there are still some things WinRAR can do which Opus can't. We haven't aimed to entirely replace dedicated archive tools & everything that they do; just to make the common/important tasks easy & integrated.
Cheers Leo. After asking my question, I went and looked at the manual (which I should've done first I suspect) and read about needing WinRAR to create/modify RAR archives. I suppose it is wisest to leave WinRAR installed, especially if as you say it still can do things which Opus cannot. Plus, I'm probably one of the few that actually bought a license (lifetime I might add) for it back in 2006!
Since buying Opus, I've moved all my archives from .rar to .7z as the compression rates are as good if not better. Plus, I've always been a fan of 7-Zip and was most pleased to see it in-built. I'm guessing this was relatively easy given the GPL.
It was a huge amount of work, in fact. Making archive APIs behave like filesystems is a big task and there's a lot of complex stuff (caching, threading etc.) behind the scenes to make the Opus and 7z.dll APIs talk to each other in a good way for the very different ways they see the world. That goes double with the 7z format itself and its (by default) solid archives, where reading the first byte out of a tiny file might take 5 minutes because you have to extract a few GB of data before it. Opus wasn't designed to work with that sort of thing so in Opus 10 we had to make lots of changes to make sure things behaved reasonably in those situations. It was also the first read-write archive plugin for Opus and we ended up improving the Opus plugin API a great deal in the process of writing it; stuff which took us time to work out.
Having the source is very useful but without much documentation you spend a lot of time working out the correct way to use things. I'd often choose good documentation over access to sourcecode (the ideal is having both, of course; documentation never covers everything, but you only want to dig into someone else's code as a last resort), but I am not complaining. 7-Zip is a great thing and provides a well-designed API which abstracts a large number of archive formats, and despite being free it's better than anything you can pay for. No complaints at all, but bridging the two worlds was a huge project. (And all for something that, if it does its job properly, you don't really know is there. )
Well how wrong was I?!, lol. Thanks for adding it though, it really is very useful.
Glad it's worth it!