But Google suggests that the addons can go more-or-less anywhere, so long as they are owned by the 'odoo' user and the path is added to the conf file (/etc/odoo/odoo.conf).

I've just launched an Odoo server and after having a quick look, I have used a sub-driectory of an existing Odoo dir; /var/lib/odoo/custom-addons. Here's how I did it:

mkdir -p /var/lib/odoo/custom-addons
sed -i "/^addons_path =/ s|$|, /var/lib/odoo/custom-addons|" /etc/odoo/odoo.conf

Then put the desired downloaded addon in that directory (you'll need to unpack the zip file). Here's a commandline example showing how to download and install the MuK DMS addon (I got the download URL by right clicking on the "Download link" & copy/pasting it):

DL_URL=https://apps.odoo.com/loempia/download/muk_dms/11.0.2.1.7/1x53hdEByYvFF4wLLMCArl.zip?deps
wget $URL -O /var/lib/odoo/custom-addons/muk_dms_11.0.2.1.7.zip
cd /var/lib/odoo/custom-addons
unzip 

Then fix the permissions (so the files are owned by Odoo) and restart the Odoo service:

chown -R odoo:odoo /var/lib/odoo/custom-addons
find /var/lib/odoo/custom-addons -type d -exec chmod 750 {} \;
find /var/lib/odoo/custom-addons -type f -exec chmod 640 {} \;
service odoo restart

Note to enable and use your desired addon/App, additional steps may be required.

For example in the case of the MuK DMS module, you'll need to follow the steps as noted in the Documentation section of the Odoo Addons page. I.e.:

Hopefully you have a similar experience to me!

One final note. If you are using TKLBAM and want to include all your downloaded custom apps in your backup (rather than needing to redownload them if things go pearshaped) then add an override:

echo "/var/lib/odoo/custom-addons" >> /etc/tklbam/overrides