Is it possible to boot into a Windows bootcamp partition in Target Disk Mode?
Let me try to explain what is happening.

The MacBook Pro is put in Target disk mode. At this point, the MacBook Pro is running software (firmware) stored on the logic board. This software is provided by Apple.

The iMac boots. At this point,  the iMac is also running software (firmware) stored on the logic board. This software is also provided by Apple.

The iMac recognizes the MacBook Pro as an external drive and determines the default operating system to boot is on this drive.

The iMac reads the necessary file from the hidden EFI partition on the MacBook Pro. This file is loaded into the memory on the iMac and executed. This file is provided by Microsoft.

The executing program from Microsoft now running on the iMac has no idea what Target disk mode is and no idea how to communicate with the MacBook Pro using the cable connecting the two computers together.

So you may see Windows start to boot but in the end Windows can not read additional files from the MacBook Pro.

There is no solution to this problem, but even if there was, Window still would fail to boot. Windows is licenced on a per computer basis. So at some point later in the boot sequence, Windows would have determined the computer was an iMac instead of the MacBook Pro. Windows would then have produced an error message and halted.