You should read here,   http://rootzwiki.com/topic/8523-reverse-engineering-the-dock-charger/
The connector is a bit expensive and I haven't bothered lookng elsewhere for it.
The 3.3 volt LDO can be anyone it is just to drive the inverter and give a 3.3 reference so the the TPT can pull it down and start charging.
The inverters only demand is to work at 3.3 volts.
I chose an intelligent fet as they are easy to use and are protected in all possible ways (almost) :-)
The fet only works as a switch here. There is no pwm going on. And it switches positive side so using a P-channel fet would need some more components round it.

The build can be done using normal through hole components as well. I chose smd as I work with those all day and have the equipment.
The PSU just needs to output around 20 volts. Mine gives 19,5 volts and comes from an old Dell laptop.
If You wan't You can add an usb cable as well so You don't need to use the micro usb as well.

To charge on the road I bought a car adapter for my mini laptop and uses it to charge the TPT too.
I will build the chrager parts into one of those too instead of having a little box in the middle of the cable.

I've been working with reverse engineering for a lot of years and with prototyping.
Started with hacking and hardware stuff on the C64 and Amiga scene.
This is a real "budget" hack until now. I hope too get my hands on a broken tpt to explore other boot options as I really do not wanna break this apart.
But someday I might get tired of waiting for a root hack .....

Just have to alliance up with some more of these here in Oslo, Norway and maybe we can get the ball running.