You can make a similar set up to the Battery Tender stuff with a plug, some rings and the clips.

 

Get creative. Go to the local parts store and see what they have. I used a flat 4 trailer plug to do something similar on my tractor. The plug looks similar to that battery tender plug except has 4 ports/wires. I wired two for power and 2 for ground.

Also the bike isn't too bad to push start if it's already been running. I had to push start mine once. I ran as fast as I could then jumped onto the seat and popped it into 2nd and it caught right away. Now it had been raining but I was on a hard packed dirt surface so it was OK. I can see how push starting in mud would be problematic.

 

I added a 12v socket to my bike as well as my wife's bike. Then I made a reverse plug to connect the two in case one of the bikes got low on battery power or a key was left on. In the MSF class that my wife took they taught everyone to use the kill switch to turn off the bike. WRONG!!! I have never use it and told my wife to do the same. You hit the kill switch and the headlight is still on, you step off the bike and get distracted.....well you get the idea.

This is what I made and it works great. Tested at home. A year after buying her bike the battery crapped out. So I bought a new one and added a kick start to her bike.

But this is still a great back up.

 

I use the battery tender on all three bikes. I just swap the plug to each bike every couple of days. I'm still on the original batter on the KLX from November of 2005