I have an 8AWG power cable running from my battery to the boot. Currently it powers a single amp running in bridged mode which states a max of around 200w RMS. The amp has a fuse in it rated at 10 amps, no idea if that is what it should have, its as old as the BA itself I'm willing to bet, and can't find any info online about it. Anyway, its never blown this fuse and suits my needs fine.

The power cable came with a 60amp fuse (fitted at the battery end of course). This fuse obviously won't provide any protection to the amplifier in case of a malfunction, but would (I hope) blow quickly in the event of a dead short of the power cable preventing a fire.

Say I want to wire up a permanently live accessory socket in the boot for example or something in addition to the amp. As the title says - what would be a safe maximum working [b]continuous[/b] current on this cable? It's 6 meters long at most. 20 amps total? 30 amps total?

Ignore things like my alternator not being up to the job - I just want to know what the cable can do

Mate according to this site it can handle 73 amps. So I'd say the 60 amp fuse is perfect for it, just leaves enough headroom as well.
http://www.powerstream.com/Wire_Size.htm