If you're in a group (and only if you're in a group!), you can r-click on the enemy portrait and select oneof:
cyan lightning bolt
There are no keybindings for any of these. So that's super painful -- no way for a tank to quickly say "this is who I'm focusing on".
The Social Vendor has Republic Targeting Device
for 500 creds and requires Social Level I. (Thana has that already, and I hate PUGs, so I think Social I is easy to get.) This then puts a hard-to-see white Republic mark on a target, but if you drag the Targeting Device to a hotbar, you can then bind a hotkey to it. Note that the Targeting Device has a 5 sec cooldown.
It might also work a lot better if everyone knows their role, so (say) the Scoundrel marks which droid they're going to Slice Droid and the Sage marks which mob they're going to Force Lift, rather than waiting for the tank to mark everything. That, actually, would be a nice change.
I'm still gonna avoid PUGs like the plague, though.