Check out the various Downloads / Tutorials areas on the Opus forums for people's shared configs, advice, etc. Lots of good stuff there. There's also a very long & detailed customization tutorial soon to be posted by one of the forum members, who let me see a draft copy recently, so keep an eye out for that.
partly because the DOpus authors have finally caved on their long-time insistence that they'd never write archive plugins for either RAR and ZIP (and let's face it, their support for RAR in the past has been very weak and very much and after-thought)
I think there's some misunderstanding there.
Opus has never needed a Zip archive plugin because Zip support has always been built-in. (If my memory serves me, that's been the case since the first Windows version more than a decade ago.
(Maybe you meant 7z rather than Zip? But nobody ever refused to write a 7z plugin either. It took until Opus 10 to happen but that wasn't for lack of desire.)
As for RAR, Opus has had a RAR archive plugin since seven or eight years ago.
It's possible I've forgotten a very old statement but I don't think anyone ever insisted that archive plugins would not be written, for any filetype. Adding the archive/VFS plugin API back in 2004 (or whenever it was) would've been pretty pointless if the aim was to never write any plugins for it.
There were issues with making the RAR plugin anything other than read-only, because while the RAR decompression code is available for anyone to use/licence, the RAR compression code is not made available to anyone in the world, for love nor money, except to RARLabs themselves. We've always wished the code was available and always wanted a fully read-write RAR plugin but it's also always been impossible to write it how we wanted to.
In Opus 10 we addressed that by making Opus able to automate WinRAR.exe, if it is available, to modify RAR archives. (Extraction is still done without WinRAR, as it always was.) Automating WinRAR is a kludge and something we were reluctant to do, but I have to admit it works pretty well in the end. It's not perfect but it's better than I thought it would be. (It was me who wrote the code to do it and I was not sure, until it was finished, if it would work well enough to release, or if it would provide any real value over simply using the WinRAR context menus which already worked with Opus. In the end it did both and was worth spending the time on. I'd still re-write the code to use a RAR compression library if one actually existed, though.)
Like probably every tool except RAR/WinRAR itself, support for updating RAR files isn't as good as for updating Zip, 7z, TAR, etc. For example, some complex operations have to be done, behind the scenes, in multiple stages instead of in a single update. (While Opus handles the details and sets up each stage of the operation automatically, it means some RAR updates take longer than similar updates for other archive formats. There are also some things you can do in Opus with other archive formats that you cannot do with RAR, due to limitations of what WinRAR can be asked to do. For example, you cannot rename files on the fly while adding them to RAR archives. But, on the whole, modifying RAR archives via Opus works well. (Unless there are loads of complaints that nobody has sent in. :-))
I think Total Commander does something similar, with RAR.exe instead of WinRAR.exe. I went with WinRAR.exe at the time because it had better Unicode support (not sure if that's still the case with RAR 4) and so that any errors from the RAR program could appear in the program's UI. (The WinRAR UI is kept hidden normally, though; you see standard Opus progress dialogs instead.)
Automating (Win)RAR.exe is as good as anyone can realistically do unless the makers of RAR become willing to licence their proprietary compression algorithms as is done for other archive formats. I've long wished for them to do that, but also long given up hope that it will happen, and for that reason I still personally choose and advocate using other archive formats over RAR, but I can't change what other people want.