Yes, you can put 12V directly on the glow plugs.
It may be simpler for you to find just the one lead that throws the glow plug relay and hot wire THAT to the battery. We can probably look up the wire number for you. This assumes you are seeing 12-V on the big supply wire into the relay and the strip fuse is still good.
Doing a hot wire this way involves a lot less "jumper" current. Normally each glow plug is going to start off at 20-amps plus and drop down to 12-amps or so when it gets hot. So you would be looking at trying to jumper 80-amps to start the glow plugs.
Some of the early Yanmar diesel tractors from Japan did not have glow plugs at all to start them in cold weather. As an option, you could get a thermostart system. You had this little half-pint bottle you filled with diesel, and you threw a certain switch (position of the key I think) on the dash. This heated up something like a cigarette lighter element in the intake throat - and a line coming down from that little bottle dripped slowly on the red hot element. So essentially you had a little fire going in the intake.
Ken300D