First off, let me start by saying you're not going to like the answer very much...

In terms of commercial software, all you really have available is AfterShot. It does do tagging and organizing quite well, though, if you learn how to use it. Make sure you realize you can set up a bunch of tags in use them in the "keyword sets" dialog on the right hand side panel, so it becomes a quick handy way to tag images very quickly. I have sets for todo, family members, places, friends, online topics, etc. That way I can very quickly mark images for different things as I scan through them. EG:

If you create hierarchical tags, then you have the ability to sort later by hierarchy and find the images you want quickly using the metadata browser in the library on the left hand side. Learn to use this, as finding your images suddenly becomes very easy once they're properly tagged. And you can tag things on import too (eg, mine are all tagged Todo;Sort on import so I can find images I haven't finished sorting).

Note that these screenshots were taken with ASP1 not ASP2, but it's similar. ASP2 kept crashing and freezing on my system when I installed it so I reverted and haven't tried again yet. I probably should.

Now, having said all that: ASP is really behind the times when it comes to a decent editor. If you go look at lightroom on windows or OSX you'll find it has higher output quality and a much better filter set. That being said, I actually think ASP's hierarchical tags system is better, and the software is actually quite a bit faster browsing, cataloging and managing your images.

If I was going to start again today, you should definitely look at darktable which has come a long long way recently and might be better than ASP. I believe it is, actually, but I haven't used it extensively enough to offer a decent comparison. I will say that I believe it is much more close to lightroom, but does feel slower as well. And I still think ASP has the best of the cataloging features.

IMHO, no software offers a really good merge of everything. And I think they're all struggling with providing us the ideal cataloging interface, as none of them make it as easy and fast as it should be. I actually wrote some code for KPhotoAlbum that allows super-fast tagging while viewing the images using keys. I really really miss that ability, as what I'm using now (ASP1) isn't as fast and nice.

But the one thing you also asked: is there anything you can do that will let you tag in one system, and use them in another: the answer is definitely "no". They store tags and information in different ways and there is no easy way to get them to use each other's data, unfortunately.