Can I use individual strands in cat5 or cat6 solid core wire for pcb mods / jumper / repairs?
You can, its really great for breadboard wire. I used to use Ethernet or phone as my primary way to prototype (now I only solder). 

A better way is to use the standard "blue wire" 30AWG for jumper connections, its smaller so it's easier to attach to most SMT pins. 

There are some caveats with solid core ethernet wire though, The main problem is strain relief and breakage. As with any wire, stripping can nick the wire, and the wire breaks after moving. This is especially a problem if the wire is causing intermittent connections, it can be hard to track down. 

So be careful not to nick the wire, and if you can, provide strain relief with tape or soldering. Don't move the wire after it is placed. 

(you should also not use solid core ethernet from the wall to the computer, if it's stepped on it breaks, and if it's moved too much it breaks, use stranded instead)