As many people on here have lamented, there are a lot of mods that are, for one reason or another, only available on Steam workshop, with no other downloads from other places such as Nexus.
After much searching, I've found a way to download these without requiring Steam (or a Steam account)!
1) Locate the page of the mod on Steam Workshop
2) Copy the URL of the mod from your browser (should look similar to this one here for Yakuza Legacy: http://steamcommunity.com/sharedfiles/filedetails/?id=238624442&searchtext
3) Go to this website*: http://steamworkshopdownloader.com/
4) Paste the URL you copied into the text box (above the "Download" button)
5) Push the "Download" button (might take a while to appear).
6) When it's ready, push the "Start Download" link. You may need to select "Save File" if your browser prompts you.
7) Locate the downloaded file, and add a ".zip" extension to it (it will typically contain a different extension or just be a random-looking filename with no extension at all).
8) Unzip the module to your Shadowrun Returns UGC area (as you would with any other SR UGC).
BTW, note that some UGC gets updated faster on Steam Workshop than it does on Nexus. The main one here is Shadowrun Unlimited, which is a number of updates behind on Nexus.
Note that the "gaddons" website is originally for addons developed for Garry's Mod - so all the other instructions (e.g. using GWTool) won't work for Shadowrun Returns (but it isn't needed anyway).
Hope this is helpful to you all.
EDIT: Updated website (above) and information on "generic" Steam workshop content (below).
* Note: Previously this was this website: http://gaddons.org/workshop
, however that URL has now changed. Additionally, it was originally designed for "Garry's Mod", whereas the other one is more generic. Finally, the steamworkshopdownloader site also helpfully loads some info from the selected mod prior when it generates the download button - so it's more helpful and user friendly.
However, despite being "more generic" than the site I posted originally, the below information is still valid (though it's been slightly updated to be clearer).
A note on using this method for retrieving other UGC for other games:
This technique can, in theory, be used to retrieve and download ANY file on Steam workshop as described above (and should work for all other games too)! Woohoo!
However, this only applies to downloading
the UGC in question. How easy it is to actually install
into this other game is highly dependent on the other game. More specifically, the format the UGC file is on Steam Workshop (it won't necessarily be a zip file!) and how easy it is to install this into the game. It might be as easy as SRR, it might be another type of archive file that might just require identifying it and unpacking it, or it might require some kind of custom conversion.
On top of this, some mods (for other games) are apparently not downloadable using this method, or indeed any method outside of Steam - apparently.
So if this doesn't work for another game, I'd recommend checking in with that game's community for UGC conversion/installation tips/tools, or possibly complaining to Steam/Valve/game's publisher directly.